### Fill colors with two contour plots

Assume I have two contour plots, of functions A and B, say, depending on x and y, with one level curve on each plot.
There are two different zones in each plot, filled with two colors, like in the following:

```
Gr=Graphics();
Gr += contour_plot(A,(x,0,10),(y,0,10),contours=1,cmap=['white', 'red']);
Gr += contour_plot(B,(x,0,10),(y,0,10),contours=1,cmap=['yellow', 'blue']);
show(Gr);
```

The problem is that the resulting picture only shows the filling colors of the second plot.

How do I get a picture with each zone (from both plots) in a different color ? some kind of transparency ?

Sorry if my question is trivial,

Thanks !