So, I wanted to make some custom colours to fit with the theme of the ships I'm using (I wanted particular light colours and engine trails, especially), so I went hunting.

I found the GC3AppearanceDefs.xml in data\core, and found the following setup:

So, the first thing I noticed is that the display icon has nothing to do with the colours in the code, but refers to a set of .png files, apparently custom-made for each colour set.

The RGB set for 'enginecolor' actually sets engine trail colour, though not the appearance of the engines, so that part is easy enough to customise.

The rest of the values are a little more puzzling...

DiffuseGradientRow 1 and 2 determine the major and minor colours of the hull. 

LightGradientRow 1, 2 and 3 determine the colour of the cockpit lights, secondary lights, and engine visual glow respectively.

But what are they a gradient of?

Trying various gradients on the LightGradient Rows gives me: 1&2: blue, 3 orange, 4 white, 5 greenish, 6 orangewhite, 7  pink, 12 blue-white, 13 green-yellow, 247 bright pink, 251 orangish, 253 bright blue. On the DiffuseGradient rows 100 and 150 are gray, 255 is bright red...

The numbers appear to work a little different for engine glow (Row3), but I'm not sure exactly how.

What I want is to make a colourscheme with Red engine glow, and Blue and Green lights. With all the above, I still have no idea how to do that...