Panel line accent color is a pre-thinned enamel wash sold in a bottle with a brush in the cap. Touch the loaded brush to a recessed line and the paint pulls itself along the groove by capillary action — you place a dot, physics draws the line.
Excess wipes away with a cotton swab barely damp with enamel thinner, leaving color only in the recesses. Black is the high-contrast default; gray and brown read softer on light armor. One warning: enamel thinner can crack bare styrene at stress points, so accent over a coat of paint or topcoat when you can.
See it in context in our panel lining & decals guides, or explore the tools hands-on in The Bench.