I don't know much about cross-pollination especially in Zinnias but if a pink zinnia gets pollinated by pollen from a yellow one, it's by no means dependable that you will get pink, or yellow, or peach or orange Zinnias from the seeds that result. I think it might depend what the parents of the two plants were. Truly a gamble.
If you want to be sure of getting a mix of colors, next year buy seed packets of one color each i.e. a packet of pink, a packet of yellow, a packet of red etc.
When they say "mixed colors" on the packet, the seed company really doesn't know for sure what they're putting in there either. I'm sure the giant hopper of seed that the packet came from had seeds of many colors but who knows what gets into each packet.
