Hi, im also doing a survey on tiles currently. Based on what I know from the shop asst, there's not much different on grade 1 & grade 2 tiles. Only grade 2 tiles might be having different color for different batches.
Grade 1 tiles will not have this discoloration issue. Just need to make sure that u order enough tiles for your house (and hopefully these are in the same 1 batch of colors).
Most of the tiles in the market are using nanotechnology, which is anti-stain. Again, it'll be up to what types of tiles that you select.

This post has been edited by cvshi515: May 30 2011, 08:34 PM