Thank you so much!
I tried to follow your advice, I'm sending you a sample. The only problem that I have concerns filling the little white part between the blue and the yellow zone, and the upper yellow-green-red zones. I didn't succeed yet in doing that, may you help me in fixing also this problem?
Thanks you so much,
Bruno
RegionPlot[{1 < x^2 + y^2 < 4, 1 < x^2 + y^2 < 4 && -2 < x < -1,
1 < x^2 + y^2 < 4 && 0 < x < 1/2, 1 < x^2 + y^2 < 4 && -1 < x < 0,
1 < x^2 + y^2 < 4 && 1/2 < x < 2}, {x, -2, 2}, {y, -2, 2},
PlotStyle -> {None, Directive[Blue, Opacity[0.4]],
Directive[Green, Opacity[0.4]], Directive[Yellow, Opacity[0.4]],
Directive[Red, Opacity[0.4]]}]
