What happens is that the plaster should "suck" the paint of the brush and if brush is too wet the paint would just sit on the suface. You can use a little rug to swipe the brush of the excess of the paint everytime you load the brush, but I found that this way you can not control how much paint is sill left on the brush.

It takes some skill to polish the intonaco before painting, my plasterer spends about 20+ minutes doing just that, however overtroweling is bad, so go figure...

So when the plaster is done right and your brush has just right amount of paint(not too wet and not too dry)
You will get that polished look you want. Also you can buff your work as you go along with dry soft brush< I use guilding brushes for that.

I also add lime (bianco di san giovanni) to all of my colors - this way the pigment would be "inside" the plaster. If you want to have paint without any white, than you need to use a "milk of lime" instead of water for your mixes, I believe thats the name.

Get some(50/50) putty stirr it with water, let it sit for a while. Lime will settle on the bottom and the clear liquid is want you whant. (sit for a while I mean a few month being stirred ocasionally)

ilia