It looks like you've got the metallic setting of the paint up high.
With metallic paint we use an environment map and different lighting to the non-metallic paints. You can change the amount of metallic look to the paints using the slider at the bottom of the colour-picker.
In the attached image the top stroke was done with the metallic slider at 100%, and the lower stroke was done with metallic at 0%. Both the strokes use the same colour, and the same brush settings.
If the slider isn't visible, try making ArtRage full-screen, then back to normal.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"