Where a card says "Pay," that indicates a Cost. Imperial Order is a good example of this.
However, with Spell Cards that require you to pay in Life Points (e.g. Dimension Fusion, Brain Control, Premature Burial, Final Countdown, etc.), this can be bypassed with Spell Economics.
The Life Points paid for Card Effects don't count as Life Points inflicted to the Player or taken by the Player, so if you activate Rainbow Life and then pay 4000 Life Points for Wall of Revealing Light, you won't regain the Life Points because they are the cost to play Wall of Revealing Light.