i had trouble with this for ages... in the end i stopped using washes in the traditional sense, as it darkened the tone of the base colour too much - i now use "pin washes" (basically a wash but in a small area - think more like "wet" lining in - does that make sense?) so i keep the brightness of the colours...
the other option is; knowing that the wash will darken the tone of the base colour, use a lighter base colour than you normally would - cos you know once washed it will end up darker...
it depends on how you like your vehicles i guess and is a matter of personnal taste, like that old chestnut of "scale colour"
rob
p.s. - the basing is excellent!!!!