I'm trying to process a picture of Jupiter from Junocam and I'm trying to align 3 B&W picture to make an RGB picture but it's not working, I tried differents opacity but it still not working, I also tried with a random image from blender and I exported each colour layer then I aligned them in a new picture but it was not working, what am I doing wrong ?
You have 3 grayscale layers (R=G=B). You have to reduce them to their color component (R, G=0,B=0), (R=0,G,B=0),(R=0,G=0,B).
You can do this for each layer with Color>Components>Channel Mixer, in which you will for instance set the Red layer to 100% of its red component and 0% of the two others. When this is done your layers should be black and red, black and green, and black and blue.
Then you add them by putting the top two ones in "Addition" mode (the mode selector is the drop down at the top of the Layers list (usually set to "Normal")).