How would you brush a thin wash over a impressed design to get some color into the design to make it show up better? Thinking of diluting color with clear vanilla. Design will be white fondant so really won't show up without a wash. Cake covered with light lime green fondant, so thinking of a thin wash in the same color, but don't want it as dark.
Any suggestions?
j.

I would do exactly as you are suggesting. If it doesn't look dark enough, then re-apply.
However, I have one more suggestion: I just did a project today where I wanted the impression to show more, so I outlined it in various edible marker colors. I wanted it multi-colored, but this may work for you if you just use the one color. You could even do the wash, wait for it to dry and outline it with edible marker, as well. Good luck!