If it's the colors themselves that give you problems, try picking up a color wheel. It helps to see actual samples of colors and where they are placed in relation to one another.
Complimentary colors are opposite one another on the wheel and provide high contrast (blue and orange, red and green, purple and yellow); colors separated by one color also compliment one another (yellow and red are separated by orange, purple and green are separated by blue); colors that are side by side create a soothing effect by appearing to blend (red and orange; green and yellow).
There are also plenty of books on color theory that could help you out. Check at your LQS for a color wheel and/or the color theory books.
Darren