Carlos,
I made up a function (since you did not post one)
ff[x_, y_] := Piecewise[{{1, 10 < x}, {0, 10 > x > 5}, {-1, x < 5}}]
Now do a color function that takes a value and determines the color.
colorfun[x_] := Which[x == 1, Red, x == 0, White, x == -1, Blue]
and your density plot
DensityPlot[ff[x, y], {x, 0, 15}, {y, 0, 15},
ColorFunction -> colorfun, ColorFunctionScaling -> False]
will plot something which looks like the French Flag.
Note you MUST use ColorFunctionScaling->False so the color function is not rescaled -- you want the colors you specified exactly.
Regards,
Neil