It's not a fluke, nor does it have to do with the height of the paint.
It's something I've noticed just recently, though it's possible it's always been like that and I just never noticed...

Example: I'm shading some part of my painting and looking to get the right shade... I put a little brighter colors down in one area, maybe some darker ones near the bottom.. I decide I want the brighter area to be more like the default neautral area... So I pick that color with the Alt key.... I plop it down and because it's on top of, or neighboring the color I just chose, it's clear to see that it's actually a pixel's worth brighter on the brightness scale....

I tested this by working out an even distribution on a large area of a single color.. Picked it, and painted it down again... Picked again and laid more paint down evenly.... after a few cycles of this, the color I'm working with is considerably brighter, and if continued for a while, I would surely end up with white paint.

Anyone else familiar with this issue? I'm working on Artrage 2.5, and though I want to upgrade to Studio Pro, I feel like I don't want to have to relearn the interface or change anything until I'm finished working on my current piece.

Thanks in advance for the help.
Keep painting!