I read somewhere about lightening your colors for scale factor. I can understand why you would do this. But what is the best way to lighten your colors. Is there a rule of thumb?

I use Polly Scale's 'aged white' , 10-20% worth. It is not an exact science for me. I add what I need until it looks right to me. In fact, all my paint mixing is sort of ' a bit of that, a dash of this'. Hope it helps.
capnjock

Be careful adding white to some colors ...I find when lightening any green color ,white has a tendency to make the green go gray...so I use a yellow or sand color to lighten these colors ...works great for OD too.

Rick

I have read that it also depends on the scale. If you were to build a full size model, then there would be no need to lighten the colour, but the smaller you go the lighter it needs to be.

I think for 1:16 it was 5% or no lightening, 1:35 was about 10%, 1:72 20%.

I suppose these are all estimations anyway, but the result does look good. Remember different whites can change the colour differently, I.E. some will lighten more for the same percentage used. And as Richard said, another colour might be better to lighten with.

Alternatively you could prime with a light colour and cover with light coats of your main colour till you get the effect required.

Ross