The Atomium stamps have been designed by the famous Belgian stamp designer Mr. Leon Janssens. Mr. Janssens has designed 100s of stamps. Each of them are known to have Artist proofs, which are typically signed and dated by him. These artist proofs are printed with different colors than issued.

They are rare to be offered and hard to get (high selling auction prices).

Up to now, I have only found artist proofs of the 4F stamp in black and red color. There should be at least artist proofs of the 2F, 2,5F and the 5F available (tbc):

The following set of proofs are also known but are just unique: