How to change the color of some irregular graphics in an image

I just have a map with different colors to represent different communications like this:

Now I need to change one of the communications’ color such as I just want to let the Old Town in the color red.How can I do this?Sorry for my poor English.

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In theory you need loadPixel and pixels

see reference

You for loop over the array and compare the color of each pixel to the color you want to replace

Then replace the pixel with the new color


save image

Very slow so you might want to do this in a new sketch and save to hard drive (and not do it every time your sketch starts)