In The Crew, if I do not have a color, do I have to play a rocket?
Looking at the rules here it says (emphasis mine).

There are five card suits: pink, blue, green, yellow, and rocket
cards. Following suit means that each player must &quot;follow&quot; the suit
choice of the first player; i.e., you have to play a card of the same
type. Only if you do not have a card of this suit may you play a card
of a different suit.

and later is says (emphasis mine)

During a color-suited trick, a rocket may only be played if a card
in the color suit cannot be played.

Reading this sounds like it follows expected playing of trick taking is option.  If you can not follow suit you may play a card of a different suit.  You may only play a rocket if you can't follow suit.
Neither of these situations say must so if you can't follow suit and have the choice of playing a rocket card or not that is a free choice.