Won't be difficult, lots to choose from if you use annuals.

Antirrhinums (have I spelt that correctly? Let's say 'snapdragons' instead!) come in a whole range of colours and are easy to grow. Another annual I love are cosmos, and these come in a whole range of colours.
If you started with things like this, you could interplant with seed sown dahlias, as these also provide a wonderful range of colours right to the end of the season. (and you can lift the tubers to do it all again the year after.)
As Bamboo says, there are so many to choose from, I'd have a good look through a few catalogues first.
But why on earth do you want to limit yourself just to one species?

Doesn't this depend on when you want the plants to be in flower?

I could suggest Penstemon and Phlox for summer - autumn flowers. A bit dull the rest of the year, though!

Exam question? Possibly?

You could be right, Bamboo.

they would be seasonal. but violas/pansies. primulas and then there are roses.

Dahlias, get my vote!