Paint the grooves any way you like, then sand the paint off the top.
Or put some kind of film on the top (like spray-mount paper, wax, or
shellac) before routing (perhaps the pattern for the routing?), then
spray paint, then peel off the film (or sand, for shellac or wax,
assuming the paint doesn't just peel off it).
If it were me, I'd be using a paper template anyway, for the routing,
and I'd tack it on with a light spray adhesive. After routing, I'd
spray, then peel off the template, then sand the top to clean up the
For poured paint, why not tinted epoxy?