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I want to symbolize several values from one layer using R, G, and B values. So, column 1 would be the blue aspect, from 0-255 based on min and max values, column 2 would be green, and column 3 would be red.

I can't find a way to do this in QGIS 3 without creating 16 million rules. If I make multiple layers with color and transparency, then whichever layer is added last dominates the color spectrum.

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You can use an expression to control colour, for example in the Simple Fill of a polygon, click the "expression" button at the right hand side, then choose "Edit":

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Then in the expression enter something like:

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where "red", "green", and "blue" are the field names with the colour values from 0 to 255. Put the names in double quotes as shown. Help on the color_rgb function should show up in the dialog. Apply that and...

enter image description here

polygons coloured (and labelled, so you can tell) with RGB values from the attributes.

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  • Those color helper functions are really helpful. I am trying cmyk so that I can change brightness independent of shade (otherwise everything comes out pastel with my data)
    – user15741
    Commented Jun 14, 2019 at 16:21

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