There are several functions for drawing lines, curves and fills.
You can use the WinXDraw_GetColour function to get a colour from the user
WinXDrawLine
WinXDrawBezier
WinXDrawArc
WinXDrawEllipse
WinXDrawFilledEllipse
WinXDrawRect
WinXDrawFilledRect
WinXDrawFilledArea
WinXDraw_GetColour