Good to see your project is comin' along!
As for the colors, there's always a trade off. What looks best with the LEDs on, likely won't be the same as what looks best if the LEDs were off. Do ya want good contrast between the background and the characters to make 'em pop when the LEDs are off? ...or do you want it more subtle unless it's lit? While there can be good arguments posed for either way, what might be the best way to approach it would be to wait until you get the lights installed and then let that dictate the background color. I'm guessin' the darker colors might change the hue of the reflected light and might not give off the exact color(s) you're goin' for.
On a side note: I've been thinkin' about what my next LED project will be and I'm thinkin' it will be a large Q*bert for one wall and a giant Coily for another wall (the purple coiled snake... yea, I had to look that name up lol).