Convert contours to false colour map

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Hi everyone, first time dabbler in Opencv - enjoying it immensely so far!

I hope someone can point me in the right direction, I'm trying to work out which function would be appropriate to convert the steepness of contour lines (how CLOSE they are together) into a color/heat map.

See the following image. Idea is when the rings are close to each other they get a warmer color, cooler if they are further apart.

Any suggestions?

Cheers, -Mike