Oh no, they're not random. I think I understand your problem.
The 3DSPaint's colour selector is based heavily on the RGB model, which might take some getting used to. It uses the primary colours of light--red, green and blue (hence "RGB")--and combines them to form a single colour. The app's colour bars correspond to that exactly; low on the bar is low intensity, high on the bar is high intensity.
So if you want bright red, tap high on the red, and tap low on green and blue. If you want purple, tap high on the red, low on the green, and high on the blue. If you want white, tap high on all three bars. And so on. The fourth greyscale bar obviously makes things easier. You can check the numeric RGB values on the lower left-hand corner for some precision.
Also, a little trick I find helpful is to make the desired colour, then draw a smudge on one edge of the canvas, repeat if necessary to make a little custom palette (can be easily wiped-over later). Enter the beauty of the blue dropper tool, which picks up whatever colour you tap on your canvas. Thus you can "save" a few colours for later or immediate use.