Off the topic a bit, are there any decent diffused RGB LEDs? The ones I have all clearly show the individual LEDs' colours and no decent "mix". The ping pong ball trick wasn't really a success with mine.
So mine are ok as indicators, where I may want a red or green as an indicator for example, but no use for any sexy lighting effects.
How is it off topic? This is the forum section for leds. 
Why do you say the ping pong ball trick did not work? You can still see the separate leds through the ball?
One trick I have heard, but not tried, is to rub the led with sandpaper
How is it off topic? This is the forum section for leds.
The original post was split from another unrelated topic in another forum section and moved here
One trick I have heard, but not tried, is to rub the led with sandpaper
It will work if it is a 5mm LED, but the surface mount WS2812 not so well. The problem is that there is not enough distance between the light source and the diffuser. You need about 8mm distance for good results.
Sandpaper on perspex ( plexiglass ) or perspex with a frosted finish are good. As is polyester drafting film, greaseproof paper, and very thin 0.5mm styrene sheet.
You must be doing something wrong with the ping pong ball because that also is good. Just mount the LED in the center not against the side.
Or maybe there are pinpong balls and pingpong balls, differing opacity perhaps. Next time I will try to find non-clear LEDs. I don't buy them often.
I think the problem not solved by the ping pong ball, is that the rounded end of the LED encapsulation is a lens, and focuses the light from the colour chip in a certain direction so it projects the (blurred) image of the separate chips themselves to separate areas of the ping pong ball.