Animating simple diffuse colour (procedural)

I’m working on an animation where I want to animate a textureless backdrop plane to act as sky. As an example, it should fade from light blue (day), to red (sunset) then to black (night).

When I try to animate the diffuse material, tow process appears to work if I right click the “diffuse square” under the Material tab. However, once I keyframe a few colours, I realise that Blender is screwing up previously keyed colours as I each time I key a new one.

Either it’s a Bug, or I’m not getting the process right - how can I animate a straight material (Blender Internal) to fade through the specific colours I want?

I think I might know what’s going on. With right-clicking and keying the colour square, Blender is only keying the top value (Red). It seems to work if I go into the colour picker and separately key each of the R, G, B values.

I think this is a UI bug.