Why some of your colours never reach the screen

Set Material repaints everything beneath it. Put one below a Join and it repaints every branch the Join gathered, so all the colours you set upstream are replaced by that one.

This hides well, because the result is plausible. On a door built from panels, glass, glazing bars, a handle, an escutcheon and a hinge strap, a single leaf material below the Join painted all six the same brown. Six of eleven colours never reached the screen and every one of the eleven swatches still sat in the knob panel looking live.

Paint each branch before the Join, never the Join's output.

The graph doctor now flags this and names both ends, the colour that is lost and the material doing it. The knob panel also stops drawing a swatch that cannot reach the screen, so a colour knob you can see is a colour knob that works.

If you are checking a model yourself, count colours on the finished object rather than counting the materials in the graph. An audit that counts materials that exist will pass a model whose colours never arrive.