Why do alkali metals form unipositive ions and impart characteristic colours to flame?

The ionisation energy of alkali metals is very low. The energy of the flame is sufficient to excite the electrons to higher energy level.

The excited state is unstable. Hence, the electrons emit energy and come back to the original levels. this emitted energy falls in the visible. the region of the electromagnetic spectrum. this is why alkali metals impart colours to flame.