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I am plotting the best response functions in a basic quantity setting Duopoly model using the ParametricPlot function. The issue is that the resulting graphs are both "black" even thought I have specified the PlotStyle -> {{Blue,Thick},{Red,Thick}}, and the AxisLabel -> {q1,q2} option is also not be working. Any ideas on what I might be doing wrong here?

BRq1[q2_, c1_, a_] := 1/2 (a - c1 - q2);
BRq2[q1_, c2_, a_] := 1/2 (a - c2 - q1);
ParametricPlot[{{q2, BRq1[q2, c1, a]}, {BRq2[q1, c2, a], q1}}, {q1, 
0, 100}, {q2, 0, 100}, PlotStyle -> {{Blue, Thick}, {Red, Thick}}, 
PlotRange -> {{0, 100}, {0, 100}}, AxesLabel -> {q1, q2}, AspectRatio -> 1, 
PlotLabel -> Style["Best Response Curves", Blue, 20]], 
Style["Parameters", Bold, Medium], {c1, 0, 50}, {c2, 0, 100}, {a, 10,130}]
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up vote 3 down vote accepted

About Axis, for two parameters th default is Frame so use FrameLabels.

If there are two parameters then MMA expects some kind of surface, this causes confusion. What you see is not the "plot" but the boundary of this expected region. You can fix this with:

Mesh -> None, PlotStyle -> (EdgeForm /@ Directive @@@ {{Blue, Thick}, {Red, Thick}})

enter image description here

I'd go with one parameter:

 ParametricPlot[{{q1, BRq1[q1, c1, a]}, {BRq2[q1, c2, a], q1}}, 
                {q1, 0, 100}, PlotStyle -> {{Blue, Thick}, {Red, Thick}}, 
                PlotRange -> {{0, 100}, {0, 100}}], 
 {c1, 0, 50}, {c2, 0, 100}, {a, 10, 130}]
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