If you want to add translucent color, try the
Minitare Ghost Tints and then seal them with gloss varnish (The link is for the Warstore's Minitaire section. Just type in Ghost Tint into the search field.). They're basically the acrylic version of auto candies and while designed for airbrush use, they go on just fine with a regular brush and are available in a dozen or so colors. I usually dilute them at least 2 parts water to 1 part ghost tint.