GIMP :: How To Make All Blue And Red In Image Of One Plain Shade

Referring to the image attached, this is my question: I want to make all the blue and red in the image of one plain shade. You can see that in the image there are couple of patches of red and blue color separated by while lines. Now inside one of these colored patches (which is isolated form another patch by a combination of above mentioned white lines and the boundary of the image itself), the color is either red or blue. This color does not look uniform because this image is a scan of a painting. Hence, although the entire patch is red/blue, not every pixel in that patch is of the same shade. If I wanted to change everything inside one such region to a plain red or blue color, how can I do that? If such is the case then I will try to articulate it better. Find attached, the image.