Colour is the one line on a coated-steel order that everyone reads and almost nobody specifies. “Grey” survives the enquiry, the proforma invoice and the production schedule, then turns into an argument at the container when two batches land two shades apart. Prepainted steel is sold by the tonne and judged by eye, which is why the colour line needs more words than the grade line.

A RAL number is a reference, not a recipe

The standard colour line on TDC51D+Z PPGI reads “RAL Color or as customer’s sample”, and that is really two different contracts. A RAL number fixes a target appearance against a published chart. A physical sample fixes it against a chip of painted metal that will be handled, posted, and left on a desk in daylight for six weeks before production starts. Both work. They place the risk in different hands, and the second one places more of it with the buyer.

Neither one says anything about durability. That sits with the resin system and the film build. The PPGL and PPGI catalogue lists surface colour as RAL plus wood grain, camouflage, stone, matte, high gloss and flower patterns, produced to EN 10147, EN 10142, DIN 17162, JIS G3302 and ASTM A653 on hot-dip galvanized, hot-dip galvalume or electro-galvanized base metal; the JSW PPGI listing carries the same standard set. The same page notes that coatings are chosen by the environment the resin is selected for — polyester, silicon modified polyester, polyvinyl chloride plastisol, polyvinylidene chloride. Colour is the visible half of that decision; resin is the half that decides whether the colour is still there in year twelve.

The back coat is a different product

On DX52D galvanized corrugated sheet the specification is explicit: the top side is made to the RAL colour, the back side is white grey as standard. Nobody argues about the back coat, which is strange, because it is the surface that meets condensation, insulation and the inside of a building.

It matters in three places: unlined roofs and walls where the underside is also visible; sandwich panel lines, where the back coat is the bonding face and an unknown primer is a delamination risk waiting to be found; and any sheet roll-formed on a dirty or unlubricated machine, because the back coat takes the abrasion. Specify it in microns like the top coat. “Standard” is not a thickness.

Gloss and texture change how the building reads, and how it gets repaired

Wrinkle PPGI is made by continuous roller coating with a topcoat carrying special additives, then baked so the film pulls into a fishnet texture. The supplier will customise pattern size, shade and colour. It runs 0.12 to 4.0 mm in the usual widths of 914, 1000, 1200, 1219, 1250 and 1220 mm, on coil IDs of 508 or 610 mm, over a zinc coating from Z40 to Z275, at 200 to 550 N/mm² tensile and 16 to 30 per cent elongation.

Texture earns its keep on wide wall panels, where it hides oil-canning and roll-forming marks that a smooth finish broadcasts. It costs you three things. It traps dust near a road or a quarry. It makes thickness gauging over the film unreliable, so you verify the substrate thickness on the certificate rather than on the sheet. And it makes touch-up impossible with a brush — a sprayed repair on a fishnet surface never matches, so the repair strategy has to be panel replacement, agreed before the first sheet is fixed. High gloss behaves the opposite way: it shows every flatness defect, and it holds its colour longer.

Printing, patterns and the per-metre logo

Branding is a separate operation. On Classic Rib galvalume panels the standard is one logo per metre, with the colour and design open to the buyer. That is a printing pass with its own ink, its own registration and its own approval sample, and it needs the same discipline as the paint: a signed-off printed sample, not a PDF.

Pattern coats — wood grain, stone, camouflage — are printed rather than painted, which introduces repeat length. If the repeat is not agreed, two panels cut from different points in the coil sit next to each other and the pattern steps at the join. On a long elevation that is the most visible defect a coated building can have, and it is entirely a specification failure.

Colour is a commercial variable, not only an aesthetic one

Every colour is a line setup, and every line setup has a minimum. Wrinkle PPGI carries 15 tonnes per colour with delivery inside 30 days. DX52D corrugated sheet is 25 tonnes, sold as one 20 ft FCL, at 15 to 20 days. Pre-painted aluminium-zinc coil is 5 tonnes minimum with 7 to 14 days after order confirmation. Coil weights differ too: 3 to 20 tonnes on PPGL, 3 to 5 tonnes on pre-painted Al-Zn, 3 to 10 tonnes on wrinkle product.

Work that out before the elevation is finalised. A four-colour facade is four setups, four minimums, four sample approvals and four chances to get a batch wrong. Dropping to two colours usually costs less than the money saved with a cheaper finish, and it halves the risk of a shade break between phases.

Dark colours carry a heat penalty

A bare galvalume surface reflects up to 80 per cent of sunlight. Paint a deep charcoal matte over the same substrate and you have built the opposite object. Panel temperature climbs, thermal movement follows, and the movement has to go somewhere — into fixing slots, clip spacing and end laps. Chalking accelerates, and the cooling load goes up with it.

None of that is a reason to avoid dark colours. It is a reason to specify substrate and resin together with the shade. AZ150 aluminium-zinc coating is rated for roughly 25 years of normal service life and withstands more than 300 °C; where exposure is marine or industrial, coating mass and substrate do more for the building than the colour chart ever will. At the light end, cold rolled aluzinc coil and a Z275 galvanized base are the usual pairing, with electro-galvanized SECC quoted where a formed part needs a smoother, thinner-coated surface.

How to write the colour line

Six items: the RAL number, the gloss range at 60 degrees, the resin system, the top-coat film thickness in microns, the back-coat specification, and the coating mass on the substrate — Z275 or AZ150, not “galvanized”. Add the substrate itself, because the same colour over hot-dip zinc and over 55 per cent aluminium-zinc does not weather alike.

Approve on a production sample rather than a printed chart, and keep two retained panels per batch in the dark. Printed charts fade, screens lie, and a retained panel from the actual run is the only reference that settles a dispute. State the batch rule up front: if an elevation has to be one colour across two phases, order it as one production run. Roofing sheet and cladding also run through the galvanized corrugated and GI and PPGI corrugated ranges plus the processing line, so profiles, cutting and bending settle on one order rather than three.