A node that takes four numbers (int or float, doesn't matter) as R, G, B, and A and outputs a color is needed for situations where color is driven mathematically by other elements.

The best workaround I have found when alpha isn't needed is to create three gradients, one for Black->Red, Black->Green, and Black->Blue. I can then sample those gradients and use them to produce three duplicates of the geometry, each representing one color channel. Finally, I can blend them using the Lighten blendmode to get the composite image.