Colormap of histogram affected by contour plot on same axes

I am making a histogram2 (tile view) of one dataset, which is time samples of two variables. On the same axes I plot a contour of a map which comes from different data (essentially a target map for the time based data which is in the histogram).

The unusual behaviour is that the colour scale of the hist2 appears to be affected by having the contour on the same axes.

If it is noteworthy, I am plotting the contour first, with a solid colour for all contour lines (not using colormap). Then I plot the histogram second.

If anyone knows how to resolve this, I would be most grateful.

Many thanks