I won't tell you how to use QColorDialog, but the Qwt related question is how to assign a color to a symbol of a point:

If your symbol is a point of a curve you can't change its color without changing the color of all other points of the curve ( beside you overload QwtPlotCurve::drawSymbols ). If it is a marker you can simply assign the color to the symbol. If you don't have too many colored points you can have your point as part of the curve + as a marker with a higher z value.

Uwe