This is a picture of a new forming Opuntia spec. disk. These young disks still have leaves (the green things), which will dry out soon. The color of the disk itself will turn to green.

I found these artificially colored roses at the Hortifair 2007 in Amsterdam. The person at the stand was not allowed to tell me how they are produced, but a fair guess would be that they split the stem into a few parts, and put every few vascular bundles in a different color solution. Each petal is linked to a few specific vessels, so this way this effect can be achieved. Ofcourse this is only speculation...

This plant has a huge flower, larger than the plant itself. The flower is open for only 1 night, and makes your greenhouse smell like a candy store!