Ask a moulding shop why a certain part is only available in three colours and the answer is rarely about taste. It is about the barrel. Inside an injection-moulding machine a screw melts pellets and pushes the melt forward into a mould. When a batch ends, the barrel is not empty: a film of melted material and a quantity of colorant cling to the screw, the check ring and the walls of the heating zones. Start feeding a new colour and the first several hundred shots come out streaked with the old one. To get a clean result you have to purge, which means feeding new material through until the discharge is uniform.
Purging costs three things at once. It consumes resin that becomes waste or downgraded stock. It occupies the machine for hours during which no saleable parts are made. And it needs an operator to watch, judge the colour by eye against a sample, and decide when to stop. On a small part with a high cosmetic standard, a dark-to-light change can eat most of a shift. A light-to-dark change is much cheaper, because a little dark pigment covers a pale residue within a few shots. The direction of the change matters as much as the change itself.
Why the schedule is a colour gradient
Once that asymmetry is understood, the production schedule stops looking arbitrary. A planner groups orders so the line runs white, then cream, then yellow, then red, then black, and avoids going back up the gradient until the next full cycle. Holding the same mould for a whole colour family means one purge instead of five. The rule is written into the schedule as a cost, not as an aesthetic preference, and a rush order for white parts in the middle of a black run is expensive precisely because it undoes the gradient.
This is one reason a manufacturing district stocks parts rather than finished lamps. Colour is one of the last things decided. If the same unpainted housing serves six finishes, the moulding shop can run long, uninterrupted batches and the colour is applied further down the chain in a smaller, quicker process. Decide the colour early and you commit the whole barrel to it; decide it late and you keep the option open. A toy district that fits on one street works on the same logic, with the mould shop, the moulding machines and the packing table close enough that a change of plan costs a walk rather than a truck journey.
Why two batches of the same colour do not match
The purge also explains a complaint that customers often read as carelessness: two boxes of the nominally identical product showing slightly different shades. Colorant is dosed as masterbatch pellets mixed with natural resin, and the result depends on how completely the previous pigment was purged, on the actual dose, on melt temperature, and on how long the material sat in the barrel. A trace of a warmer residue changes a pale grey more than a deep one. Add a different lot of masterbatch or a different supplier of natural resin and the same recipe drifts again. Matching is a controlled approximation, not a fixed identity.
Where the old pigment hides is predictable too. Dead spots behind the screw, around the check ring and at the gate of a hot runner hold material that moves slowly, so they keep releasing colour long after the main melt has turned clean. Shops sometimes run a dedicated purging compound that swells and scrubs those corners, then follow it with the new resin. On a part with a long, narrow flow path the last streaks appear near the gate, which is why small thin-walled pieces expose a bad purge sooner than a thick panel does.
Shops handle this by keeping reference plaques, quarantining the first shots after a change, and grading parts into colour families before packing. That grading is another argument for keeping the whole line in step: once a colour run starts, every downstream station has to be ready for that colour, because a half-purged batch held in a bin for a day will not be the same shade as the next one.
The material pushed through has to go somewhere. Some is reground and fed back into dark or hidden parts where a slight shade drift does not matter; some is sold as lower-grade stock; the rest is scrap. That is why a factory with a short colour ladder usually has a tidy regrind system as well. The waste is not an accident discovered at the end of the process — it is a quantity the planner estimates before accepting the order.
None of this shows up on the product. A customer sees a row of identical little parts and assumes the only difference between them is a choice made in a design meeting. In fact that choice determines how the factory spends its morning, how much scrap it produces, and whether the order can be fitted in before the weekend. Colour is one of the most expensive decisions in the building, and it is made at the very end of the process.