I downloaded a model from BlenderSwap - https://www.blendswap.com/blends/view/88280 - but when I render it the result is almost colorless but not quite (so I am pretty sure it is not greyscale, although have double checked the render settings and they are on RGBA
One of the materials in the model called Clay does seem to be this color but most of the faces are assigned the yellow or green materials which show up fine ... until it is rendered.
There are no textures in use so it can't be missing external files.
I've compared my render settings with other blender files that work and they seem identical.
What am I missing?