Play sand does not have to meet any particular specification, as far as I know. It would have to be relatively clean and free of broken glass, or other unacceptable debris. But, it could be limestone and would be perfectly acceptable for its intended use, but not for a planted tank. Even pool filter sand varies widely in appearance because appearance is of no concern for that job. But, I don't believe a sand that changes the hardness of the pool water would be acceptable as a filter media, so at least you don't have that to be concerned about with filter sand. In fact any time you use something not specifically sold for planted tank substrates there is a chance it will affect the water - even SMS did that for some people. But, that is still the way to go for really cheap substrate.