If you weren't thinking of converting a black & white picture to colour by hand, how were you thinking of doing it? I can't believe you wanted Photoplus to automatically guess what colours went where and come up with a colour image for you, and I can think of no other way of colourising a black & white image other than by hand.
You have complete control over opactiy using the method given above. Different colour on each layer, alter the opacity to suit your tastes.
Also, when colourising, don't use the brush on 100% pressure, give it something really low, say 15-25%, and gently build up your areas.
And last but not least, remember its the layer that needs to be set to colour mode, not the brush! (you get pants results doing it that way!)
Thats good news then! Glad you got it to work how you wanted it to!
As for practicing, can't fault you! Layers are what its all about! Being able to select areas is also a valuable thing to learn too, maybe we may even get a pen tool in the next release of Photoplus to make it easier :-)