>>1835404For something this simple I just sampled one section and flood-filled the whole thing.
In general though, the eye doesn't notice such color-shifts unless they're very abrupt.
That's why shadows often appear to be darker at the edges than near their middles. It's the discontinuity.
Use blurring to eliminate sharp transitions or semi-transparently clone image A a little ways over image B.
You might also be able to adjust lightness or hue before making the actual 'join'.
Unfortunately, I don't know what tools you have available in
paint.net.