Knowledge · Warehouse and marking

Why pipes are blue, red or yellow in the warehouse

Walk through a pipe warehouse and you see black and bright pipes, bundles in blue or red in between, a yellow valve on a pallet, large grey pipes with taped ends. The colour tells you something – but not what many assume. It tells you how the material was ordered. Which colour a line later carries in service is decided by someone else.

Stock colour: ordered that way – primed or painted at the mill, usually for a project
Identification colour: set by the operator according to the fluid conveyed (DIN 2403)
Grey: pipes pre-coated at the mill, ends taped before coating
Black and bright: uncoated steel (mill scale) – the rule in the warehouse

Three sentences that make the difference

Red, blue or yellow in the warehouse usually means the material was ordered that way for a project – primed or painted at the mill. Which colour a line carries in service is set by the operator according to the fluid conveyed (identification colours DIN 2403). The two are not the same – sprinkler lines are red although water flows through them.

Stock colour: the colour is in the order

A steel pipe leaves the mill black – with the dark mill scale formed during hot rolling – or bright if it is cold drawn or ground. Stainless steel stays metallic. Everything else is the buyer’s decision: whoever builds a project specifies whether the pipes are delivered primed, painted or coated, in which shade, with which coating system and which coating thickness. The mill or a coater carries this out, and the material arrives at our warehouse in this colour, is stored like that and delivered like that.

That is why the colour in the warehouse first tells you something about the order and nothing about the medium. A blue bundle is not “water”, a red one not “steam” – it is material a customer wanted that way, often because the lines on the plant are later to stand in this colour and painting on site is no longer needed. The colour in the photo is therefore the result of one line in the order.

Blue-primed steel pipes in a bundle on the warehouse floor, uncoated pipes in a cantilever rack behind — Blue-primed steel pipes in a bundle. Photo: Zickwolff

Red-primed steel pipes in bundles in the outdoor storage area, with tags at the pipe ends — Red-primed pipes – red is industry practice for sprinkler lines. Photo: Zickwolff

Yellow-painted flanged valve, wrapped in film on a pallet for shipping — Yellow-painted flanged valve, packed for shipping. Photo: Zickwolff

Identification colour: the colour of the line in service

On the finished plant, colour has a different job. There everyone working on a line should see at once what flows through it – for maintenance, repair and in case of fire. This is what the identification of pipelines according to the fluid conveyed is for, in Germany according to DIN 2403 (edition 2025-12). It assigns group colours to the fluids and describes how labels, colour bands or continuous painting mark the line, with an arrow for the flow direction and information on the hazard. Whether the line is painted all over or only carries labels and bands is decided by the operator.

This identification colour has nothing to do with the stock colour. A pipe ordered in red only becomes a steam line in service if the operator marks it that way – and a sprinkler line is red although water flows through it, because red has always been the colour of the extinguishing line in sprinkler construction. The values of the colour groups are given in the standard DIN 2403.

DIN 2403:2025-12, “Identification of pipelines according to the fluid conveyed” (replaces DIN 2403:2018-10), available from DIN Media. The international counterpart is ISO 20560-1.

Grey with taped ends: pre-coated pipes

Grey pre-coated large-diameter pipe whose end is covered with adhesive tape; the bare steel at the pipe end is left free for the weld — Grey pre-coated pipe: the pipe end was wrapped with adhesive tape before coating – under the tape the weld area stays bare. Photo: Zickwolff

Grey pipes are usually not a choice of colour but corrosion protection: the pipe was given a primer coating at the mill; in the German trade such pipes are called AGI pipes. The coating replaces painting on site and protects the pipe from the warehouse to installation.

What stands out on these pipes are the ends: wrapped with adhesive tape or closed with caps. The reason is the later weld. Where the pipe is welded to a flange or to the next pipe, the steel must be bare – a coating in the weld disturbs welding and weld quality. That is why the pipe ends are wrapped with adhesive tape before coating; the tape covers the strip that is to stay bare. Before welding, the fitter pulls off the tape, and the pipe ends are bare and ready for processing. Anyone ordering a pre-coated pipe therefore states how long the ends are to stay bare.

Bundle of grey pre-coated pipes 168.3 by 4.5 mm, grey outside, bare inside, in a cantilever rack — Pre-coated pipes 168.3 × 4.5 mm in a bundle. Photo: Zickwolff

What is not a colour in the sense of this page

Yellow, green or blue protective caps on pipe ends are transport protection: they keep dirt out of the pipe and protect bevel and thread. Their colour depends on the manufacturer and the cap size, not on the medium. Nor do tags, labels or marking strips say anything about the identification colour – they are stock and heat markings. What is written on the pipe itself is explained in Reading markings.

Ordering material in colour: what the inquiry needs

Primed, painted or pre-coated pipes, fittings and flanges are available on request – the mill or a coater applies the coating, and the delivery time depends on it. For the quotation to be right, the inquiry should contain:

Shade – as a RAL number or according to the project specification.
Coating system and coating thickness – primer only or primer and top coat, target thickness in µm.
Bare ends – whether and how long the pipe ends stay uncoated for the weld; whether the bare ends are to be taped.
Inside or outside only – coating outside only is the rule; inside only if the medium requires it.
Lengths and quantities – random or fixed lengths, number of pieces, plus material, standard and the required type of certificate.

Uncoated material remains the normal case and is available from stock; colour is an additional service that takes time. Ask early and it goes into the delivery schedule.

Frequently asked questions

What does the colour of a pipe in the warehouse mean?

Red, blue or yellow in the warehouse usually means the material was ordered that way for a project - primed or painted at the mill. Which colour a line carries in service is set by the operator according to the fluid conveyed (colour codes DIN 2403). The two are not the same - sprinkler lines are red although water flows through them.

Is the colour in the warehouse the identification colour according to DIN 2403?

No. The stock colour is the primer or paint ordered from the mill. The identification colour according to the fluid conveyed is set by the plant operator and applied as paint, label or band on the finished line. The two can coincide if a project orders the pipes in the service colour straight away – but they do not have to.

Why are the ends of grey pipes wrapped with adhesive tape?

Because that is where they are welded. The weld to the flange or to the next pipe must be bare; that is why the pipe ends are wrapped with adhesive tape before coating. Before welding, the tape is pulled off – the ends are bare and ready for processing.

Can I order pipes in a RAL colour?

Yes, on request. State shade, coating system and coating thickness, whether the ends are to stay bare, plus lengths and quantities – the coating is done by the mill or a coater and takes time.

Why are sprinkler pipes red although water flows through them?

Because red is the established colour of the extinguishing line in sprinkler and fire-fighting systems. It marks the task of the line, not the medium – exactly the difference between identification colour and stock colour.

What does the colour of a protective cap mean?

Nothing about the medium. Protective caps are transport protection; their colour depends on the manufacturer and the cap size.

Material in your colour?

Tell us shade, coating system and bare ends – we request the coating and give you the delivery time.

Send an inquiry

Your inquiry to Zickwolff

Send us what you have – designation, standard, quantity, date. We will clarify the rest with you.

One item or a question

Send inquiry

Not in stock? We procure it – single items too.

Designation, standard and quantity are enough.

Several items

Send bill of materials

200 items? One parts list is enough – we deliver complete.

Attach it as a file.

Helpful if already known: project and country, date needed, certificates, packaging and language of the packing lists. If anything is missing, we will ask.

It is urgent

Request express

When do you need it? Express delivery, to the site if required.

Tell us by when you need the material – we will tell you what is possible.

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As of 2026-10-23. Statements from our own warehouse practice (warehouse photos of 2026-10-22); DIN 2403 mentioned by title and purpose only, no colour values reproduced. Photos: Zickwolff.