Have you tried putting your LEDs behind a frosted plate or even a slice of paper at the same distance as your project?
I have found the non-diffused LEDs make multi-colored spots .. not a solid color when not "right on" the diffused surface. You should try this asap if you have not done so yet. This will assure your design works.
You may have to add more diffusion to the LEDs to get rid of the colored spots or change the design to have the LEDs almost touching the diffusion panel (a good one). I have used hot glue and a small cut off an acrylic tube to make diffused coverings for non-diffused LEDs, just in case you try all else and still see a multi-color dot instead of a solid color dot.
If you can not find frosted plexi around or it proves ineffective you may consider glass. A stained glass supply store should have frosted glass and often a scrap bin as well. I use frosted glass even with well diffused LEDs. Glass does not scratch as easy and looks great.
Have fun