So if you are actually trying to key a flower there are a bunch of words you need to know. The colorful flower itself typically is made up of petals, surrounding the reproductive organs and in turn surrounded at the base by a sheath of sepals that are usually green and look sort of like leaves. All the petals taken together are called the corrolla and all the sepals taken together are called the calyx.
This seems pretty straightforward, and usually it is, but sometimes it can get complicated. So don’t take anything for granted! For example, sometimes other parts of can look exactly like petals—colorful and showy. Irises are an example of this. They only have three petals but it looks like they have six, three that are less showy than the others. The smaller ones are actually stamen. A lot of plants can also have sepals or leaves that look like petals. Examples of this are dogwood “flowers” or the crimson leaves of the poinsettia. In both cases, those showy displays are false petals, and the diminutive nubs in the center are the actual flowers.
Modified stamens on a Douglas iris
Flowers Of Marin 