Selvala’s last ability doesn’t care what color or colors the creatures you control are. It cares only about the power of those creatures.

Selvala’s last ability is a mana ability. It doesn’t use the stack and can’t be responded to.

Selvala’s last ability looks at the creatures you control as it resolves to determine how much mana of the chosen color to add. For example, if you control Selvala (a 4/5 creature), a 1/1 creature, and a 2/1 creature, there are three different powers among creatures you control.