Colours should be used sparingly and consistently . Garish colours should be avoided. Try designing first in monochrome, and then add colour to meet particular needs. For basic system purposes, stick to the 16 system colours - it will make life easier for you and the user. Colour combinations and the number of colours used should be thought about carefully.
Excess colour can confuse, though a lack of it can undermine motivation and interest.
Research suggests seven colours on screen is about the optimum.