Painted Rocks

I came across a bunch of painted rock activities recently and have been wanting to create the same for Agirl. It looks like it would be fun and I think she would enjoy it. The only problem I have is that Agirl still puts stuff in her mouth sometimes and I want the rocks to be big enough that she won't be able to put it in her mouth and swallow.

So what I did is this:

- Big enough river rock stones or pebble stones
- Buy non-toxic acrylic paints from any craft store - a bunch of colors

I painted both sides of the stones and dried them out in the sun. But the problem I'm hitting is that the acrylic paints don't completely cover the color of the rock itself. Either the paint is not thick enough or the stones I chose were not the right ones.

The black & red stones came out pretty well. But the white, yellow, blue and green ones are not that great. I'm hoping multiple coats of the paint will help with this but so far, no luck. I'm going to try another 4-5 coats and then give up if it doesn't work.

You can see from this photo from the black and red look good but the others don't.

Do you have any ideas on what I'm doing wrong?