Most posters designed to be illuminated come in a double sided format where the back of the poster is also printed but in reverse. That reinforces the colors and avoids some of the washed out look you are getting. See if your printer can handle that detail, and it will look that much better.
Also, it really does need to difusser, and I think the difuser as you can see the rows of LEDs.
Nice start though. Keep going.