I know that you can cross pollinate seed plants, but what if you want to mix colors, sizes and make a new variety with (For instance) an iris, is there any way to do that? I have a friend who cross pollinates tulips, but I'm still unsure about the process
Hundreds of new iris varieties are introduced every year from cross pollination. Be advised, however, that it takes several years for the seedlings to reach blooming size, and there's a great likelihood that the flower will not be particularly spectacular. (Most hybridizers select fewer than 1 in 1000 of their crosses for introduction.)
However, if you'd like to try cross pollinating your irises to develop a new hybrid, you can read and see how to do so on the Schreiner's Iris Gardens website.