How to change the colour tone of all a specific colour in a photo?

[tmanni]

You can do this like this:

1. open the 2 images in gimp
   
2. with the Color Picker Tool, pick the yellow color on the "source" image (yellow aquamarines) to set your foreground color
   
3. on the target image (green aquamarines), create a new layer and fill it the foreground color
   
4. set this layer blending mode to Lch color
   
5. add a black layer mask to this layer
   
6. paint the layer mask with white color on the green areas 
   

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Oups, I exchange the 2 images...  
Whatever the procedure is the same, but here I use HSV Color instead of Lch Color blending mode.