Fuligo septica is yellow in the mobile (or plasmodial) stage. When it stops moving and changes to the sporing stage (with purple-black spores), the brittle, outer casing over the spore mass may be white, ochraceous, grrenish, pink, dull red, brown or violet. I'm quoting a slime mould book here and I haven't seen all those colours myself. Once the sporing stage has formed I'd guess that you wouldn't get any great fading colour change, since the casing is brittle and easily broken. That is of course the natural process, since the breaking of the coveing exposes the spores for dispersal.