How to obtain superimposed colors from 3 raster RGB in QGIS

Question

I have 3 separate raster layers with binary values (1/0).

By assigning the 3 primary RGB colors to each of the layers, I would like to obtain a representation that expresses the overlap of the layers themselves with the derived colors, as in the diagram below.

I tried to reclassify the 3 layers by assigning the value 255 to the original 1 and the value 0 to the original 0, then I combined them into a single virtual layer by assigning a band to each of the 3 layers.

The result, however, is not what I expected, as shown in the first screenshot below.

Basically I get the white color where the 3 layers have overlap in the original value 1 (255) and the black where they have overlap in the original value 0 (0). However, I can't get the secondary colors cyan, magenta and yellow where only 2 layers overlap.

I can get the result I want by assigning one of the 3 RGB primary colors each to the 3 binary layers and using the "Addition" Blending mode in the Rendering Layer, as shown in the second screenshot. But in this way I'm forced to use a dark layer as background, to obtain pure secondary colors.

Answer 1

No need to reclassify the original rasters, binary works just fine. Build Virtual Raster and Merge both works here, what is important to check Place each input file into a separate band. This will allow you to use the Multiband color renderer to achieve the desired end result.

Once you created the merged/virtual raster, open the Layer Properties window.

• Under Symbology, set the render type drop-down to Multiband color.
• Select a different band (1;2;3) for each of the red, green, blue render bands.
• Set the Min values to 0; and the Max values to 1 for each.
• From the Contrast enhancement drop-down, select Stretch to MinMax.

And the result: