To avoid this, shoot every tile with the same manual settings: exposure, white balance, and processing, and include plenty of overlap. Also avoid using heavily vignetted edges of each frame.

For an existing mosaic, you need to match each tile’s tone and color: black point, white point, gamma, white balance, and any vignetting. That can be done manually with levels/curves, or more automatically with panorama stitching software such as Panotools, which can help normalize overlapping frames.

A practical manual workflow is:

• overlap layers and blend with soft masks/eraser edges
• use levels or curves to match brightness
• if needed, work in Lab color and match representative points between tiles
• retouch remaining seams with a healing tool

One community solution was to convert to grayscale, correct the tonal differences with levels, then heal the remaining transitions.

So the short answer: use consistent capture settings and overlap when shooting; for repair, match tones/colors with levels or curves, blend overlaps softly, and use stitching software if available.