I have come across several subjects of spells that I can't just find the proper color for. I've been going with my gut feeling on these. Can you think of any other color spell combinations that are cookie cutter perfect?
Here's some of mine:
Willpower* -- Yellow, Cherry Red
Oppression (to be relieved of) -- Bright Red, Orange
Victory -- off White, optional with Gold
Encouragement -- Orange
Stability -- white
I'm not too sure if my feeling for willpower is accurate.
But yes, just go with your instincts! Its what the colour means and represents to you, with candle colours, I tend to go with what the colour means to me, and the spell/ritual is just as effective. As Espirit said, you can use white to substitute anything.