Shifting RGB values of a single color channel

[Ofnuts]

If you have never coded, this is will be an uphill battle. Otherwise any language with some image library will do. Within Gimp you have the choice between C, Script-fu, and Python. C is powerful but hardcore, Script-fu is a bit alien for most people, Python is a popular language. But maybe you can avoid scripting:

• I assume that in a 18x18 weaving pattern you haven't got that many colors. So you can possibly make your pattern color-indexed and edit the relevant colors in the color map.
  
• Or with the conventional tools, for instance to shift up a channel by N:
  • Start the Levels tool
    
  • Select the color channel at the top
    
  • Subtract N on the right end if the input
    
  • Add N to the left end of the output
    
•    
  You can do something equivalent with Curves by selecting one channel and setting the output curve to a straight line which is N above the identity diagonal (shift the line up/down at one end, and set the other end to make the line parallel)
  

This said, adding linear values to 8-bit color channels has no real meaning because these channels are gamma corrected (they don't really represent a linear change for your eyes). In Gimp 2.10 you can work in linear or gamma-corrected spaces.