why not simply add two or three different textures to the pack?
No way! As I learned from all the big publishers, I have to release the other textures as a DLC after the release for a lot of money.
would it be ok to sell the psd with spots as layers?
DLC right?
I will try to just update the current pack with new textures, not selling them separately.
I prefer the gray and red paint too

Think I do like 3 or 4 colour schemes and provide them as little rusty and very rusty versions (maybe InXile has to call him Rusty Clawer).
It's good practice for me.
Please give me some suggestions.
Some more colour schemes that came to my mind were:
1 olive or sand / dark gray (military like)
2 red / white (fire fighter)
3 orange / black (ABB production line robot)
4 white / red (ambulance... hm who wants a surgery done by this guy?)
5 pink / purple (Hello Kitty) <- Even if you want this, I wont do it!
Nearly forgot to mention the "blue graphic error" on the left side.
That is the secondary lighting in the test scene, it is bluish and the main is yellow-orange thats why there is a big difference in the hue.
Next time I create some shots in a more neutral light for comparison.