Multicolor transformation

The magician shows three sponge balls in his hands. A red and a black ball are seen in his left hand and a yellow  ball in his right. The red and black balls are  placed together with the yellow one and the magician closes his hand over them. After a magical gesture he opens his hand and the three balls have magically changed into a large Rainbow colored Ball!

The normal sponge balls misures mm 50 and the big one misures 90 mm