Hi Mauro,
Expanding a little on Neil's answer, try adapting this to your needs
Manipulate[
Graphics[
{ColorData["SolarColors"][t], Disk[],
ColorData["LightTerrain"][.3], Annulus[{0, 0}, {rMin, rMin + delta}]},
PlotRange -> 40],
{{t, .5}, 0, 1},
{rMin, 30, 40}, {delta, 1, 5}]
