How can I get vibrant, smooth colors with acrylics?

It takes planning and thinking ahead. In general, you want to paint darker colors on top of lighter colors. The quick and dirty solution would be to spray a figure with white primer, and then paint everything on top of that. But that can have some downsides. Maybe you don’t want a thick layer of primer interfering with joints. Or some colors on the figure are fine as is and don’t require repainting? That’s when you have to plan and think.

I rarely apply a solid base of white, as described above. Instead, I’ll apply a “primer” coat of Testor’s Dullcote. This will apply some slight texture to the figure, giving the acrylics something to “latch” onto. (Lots of folks paint without primer of any kind, but I find that water-based paints such as acrylics tend to slide around uncoated plastic, much like water off a duck. Best to give the paint something to bite into.)

If you’re applying a dark color over a lighter color, such as black on yellow, you shouldn’t have too much trouble. But what if you’re applying yellow to… anything? Or even a dark but fairly transparent pigment like red or green? Here’s my trick: I will apply a first coat of “rosy beige” first. (Used to be called “Flesh,” but that is clearly a problematic name at best.) I use the Ceramcoat version, but there are other brands using different names. This tends to be fairly light in tone while still being somewhat opaque. Your yellow/red/green will be much easier to paint on top of this.

I also paint in thin coats. Don’t just glop the stuff on there! Even on top of “rosy beige” a yellow or translucent green can take three or four coats. I usually thin the acrylics with a drop or two of water on my brush. Thin coats might mean you have to apply more coats, but the smooth finish is worth the effort.