QPen color shift

Hi everyone!

I'm not sure if this thread should be in the Qwt subforum, but anyway, here we go.

I hace a QwtPlot, a QSlider to scale it and a QwtWheel to scroll it (both horizontally). I use a QPen to to draw in the QwtPlot. The problem is, when I scale the plot or I use the QwtWheel to scroll, the color of what I plotted changes. If there are various plots(each one with a different color), all of them change, but each one to a different color, but to same one each time.

Any ideas? I can give you more info if you need?
I didn't post any code because there's quite a lot in this.

Thanks in advance.
Alberto.