How to set default background for figures to be negative

I'm currently using matlab's rasterization functionality. After I plot a curve, I saved the whole image to a matrix using

img = print(gcf,'-r72','-RGBImage');

I set the default background as:

set (gcf, 'color', [0.0,0.0,0.0])
set(gcf, 'InvertHardcopy', 'off')

This gives [0,0,0] for every pixel in img. As I would like to distinguish my curve and the rest of the pixels, I would like to set

img(y,x,:) = [-1,-1,-1]

for all [y,x] that doesn't belong to my curve. In addition, I might have multiple curves, it would be tedious to check every curve. The optimal would be pre-settting each position with a negative value and update the positions where the curves lie.

As my curve has a varying color, it is not possible to directly mark the curve. Would there be any way to set the background like this?

Thank you very much!