Typo GBColor should be RGBColor. AspectRatio is specified twice, earlier one takes precedence.

ContourPlot[f[x, y], {x, 0, 1}, {y, 0, 2},
 ContourShading -> None,
 AspectRatio -> Automatic,
 Contours -> {0, .5, .7, .9},
 ContourStyle -> {RGBColor[0, 0, 0], RGBColor[1, 1, 0], RGBColor[0, 1, 0], RGBColor[0, 0, 1]}]

(* Here is some code that shows my problem witn=h cotour colors. The cut and paste function did not work right in my revious post *) a = 0; b = 1; c = 0; d = 2; f[x, y] := Sin[Pi x]^2 Sin[Pi y]^2; p1 = ContourPlot[f[x, y], {x, a, b}, {y, c, d}, ContourShading -> False, AspectRatio -> Automatic, Contours -> {.7}, PlotPoints -> 100, ContourStyle -> RGBColor[1, 0, 1], AspectRatio -> 1]; p2 = ContourPlot[f[x, y], {x, a, b}, {y, c, d}, ContourShading -> False, AspectRatio -> Automatic, Contours -> {.9}, PlotPoints -> 100, ContourStyle -> RGBColor[1, 0, 1], AspectRatio -> 1]; p3 = ContourPlot[f[x, y], { x, 0, 1}, {y, 0, 2}, ContourShading -> None, AspectRatio -> Automatic, Contours -> {0, .5, .7, .9}, PlotPoints -> 100, ContourStyle -> { RGBColor[0, 0, 0], RGBColor[1, 1, 0], RGBColor[0, 1, 0], GBColor[0, 0, 1]}, AspectRatio -> 1]

Adding the command "ContourStyle -> {Red, Blue, Green, Yellow, Orange} " To my code results in a plot of the contour curves that are all orange colored. They are not {Red, Blue, Green, Yellow, Orange} in that order.