Yes, I purchased empty 510 carts from HeavensGifts, they are absolutely white, not beige. The 510 carts fit on 901s, but are not as snug. They will work initially, but may become loose over time. That can be fixed a number of ways, search the forums for threads on tightening loose cartridges; here are two there's a mod where you heat a needle and push it into the cart in 4 places to tighten the fit (video on here somewhere) and you can also score around the part of the cart that fits in the atty with a hobby knife blade to tighten the fit.
I have not seen a white 901 or RN4075 cart for retail sale on any site, and I have had my eye out for them for months. I've just seen black and brown 901 carts and I think several other colors of RN4075. I have not seen white whistle tip carts of any kind, but checking Heaven-gifts I see they do have them - I guess we don't see what we aren't interested in. I've posted a number of pictures in various places of these white cartridges. Here is one pic of the 510 white cart with a 510 and a 901 brown cartridge with a 901.
