As I described in an earlier journal entry the colours produced by RB’s T-shirt printing on white shirts is quite different from the sRGB colours we see on our screens. I’ve since measured the colour behaviour when printing on grey shirts, and it’s dramatic.
To illustrate this a bit more, I’ll return to the white T-shirt case. Mapping the range of colours that can be reproduced in the “Lab” coordinate system we get shapes like this:
The wireframe is sRGB, while the solid shape is the white T-shirts. It’s easier to see the precise differences when this 3D model is rotated: this is just a snapshot. The ‘L’ axis represents lightness, with black at the bottom and white at the top. The ‘a’ and ‘b’ axes map out the range of colours.
As you can see, the response in the purples is very weak.
Now to compare the white and grey T-shirts from the same point of view:
This time the wireframe is the white T-shirt while the solid shape represents the colours of the grey shirts. Let’s look at it from the other side:
Not surprisingly, the grey shirts are much darker. At the same time the colours are significantly less saturated (closer to the ‘L’ axis). Compare the tiny space of the grey T-shirt colours to the relatively massive sRGB space in the first image!
The colours currently reproduced on grey T-shirts are very subdued.
I advise that photo printing on non-white T-shirts be avoided at the moment.
The colours reproduced on coloured shirts are just going to be weird (although I haven’t measured them).
“Line-art” can obviously be more appropriate, but even then you should be conscious of how the colours in your design will be affected!