Personally I prefer black sand. I think the sand you should choose would be dependent on the color of your axolotl. Wild type or melanistic axolotls contrast well with white sand, while leucistic and albino look good with black sand. The cheapest and one of the best looking options for black sand would be Black Diamond Blasting Sand, you should be able to pick up a 50lb. bag for around 6 dollars at Tractor Supply or Ace Hardware in the USA. Make sure for grit you get either 20/40 or 12/40 for coarse sand. 30/60 is on the finer side but should be okay. I avoid 80/40 because it is really light in my opinion and will make a mess when disturbed. Make sure you rinse the sand well before you add it into an aquarium because it does have a lot of dust and fine particles. I've used it for years in planted shrimp and fish tanks with no issues. White sand you can always go with play sand or pool filter sand you should be able to pick up a 50lb. bag of pool filter sand for about the same price as the blasting sand, just make sure it does not contain any additives to control algae. I personally like pool sand better because it is coarser than play sand and makes less of a mess when disturbed. White sand reveals algae and detritus better than black sand(which can be a good or bad thing). Black Sand tends to hide messes pretty well so just make sure you keep an eye on it for any buildup of algae or detritus.
