Let me know the version of Blender you’re using? Also what viewport shading mode you’re in. I’ll try and replicate this in a few versions – 4.2, 4.4 and the new beta for 4.5. The alpha channel on airbrushes has been notoriously buggy for a while, leaving a bit of a brown tinge in the softer areas, unless you drop the opacity of the layer you’re painting on even slightly (0.990 corrects it). This issue might be something else in the materials. If you could let me know the version, I could narrow it down and see if I can trouble-shoot.
