Hi David,

the code still exhibits the missing 'R' typo in one of the RGBColor heads. Furthermore, you appear to be using Mathematica version 5.2.

We may be looking at a bug that's 10 years or older.

It that's so, the following code may be a work-around.

Show@MapThread[ ContourPlot[f[x, y], {x, a, b}, {y, c, d}, ContourShading -> False, AspectRatio -> Automatic, Contours -> {#1}, PlotPoints -> 100, ContourStyle -> #2, AspectRatio -> 1] &, {{0, .5, .7, .9}, {RGBColor[0, 0, 0], RGBColor[1, 1, 0], RGBColor[0, 1, 0], RGBColor[0, 0, 1]}} ]

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]

Consider using the Option ContourStyle:

Clear[f];
f[x_, y_] := Sin[Pi x]^2 Sin[Pi y]^2;

ContourPlot[
    f[x, y], 
    {x, 0,1}, {y, 0,2}, 
    ContourShading -> None, 
     AspectRatio -> Automatic, Contours -> {0, .2, .5, .7, .9}, 
     PlotPoints -> 100, AspectRatio -> 1, 
     ContourStyle -> {Red, Blue, Green, Yellow, Orange}
]