Check that the brighness slider isn't all the way to the top of the brighness bar. (Labelled 'Luminance slider' in the attached picture)
If your colour-picker doesn't look like the one in the picture, click the colour-picker menu button and choose the HS/L colour picker.
Colour in ArtRage have three components 1) the Hue defines the chromacity, or actual colour. 2) The Saturation defines how rich the colour is - from dull grey to full rich colour. 3) The Luminance defines how light or dark the colour is. It's like a brightness control. With the brighness all the way down, the colour goes to black, all the way up it goes to white. To see actual colour your Luninance slider needs to be somewhere near the middle point.
And I've just realized this is an older screen-capture of ArtRage 2.0 But the colour-picker is still the same, so that should help.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"