Hi Deb! I've never heard of that before. I just did my dog with GOG and used black grout, actually same brand you have Mapie and it has not turned white at all. I've even sealed it after with Tile lab sealer and then I varnished it with a regular outdoor varnish and it's still black. Would it depend perhaps on how long you leave it with the water? I only add a few drops of water at a time until it gets to a creamy consistency and then I use it. If you do find out why, let us know for sure. Good Luck!

i'm not sure if this will help but could it be that the water you're using to mix your grout has a lot of minerals in it? same thing for the water you're filling your finished bowls with? i'm wondering if what you're seeing isn't grout at all, but minerals that have settled (which would show up a lot against black grout.)

i'd try mixing the grout powder with filtered or bottled water, in case that's the problem. maybe try filling a finished bowl with that, too, just to see.

the water in our last city was extremely hard and my black grout would turn light gray upon drying. once i switched to filtered water, my grout stayed dark.

I agree with Joy, I think its minerals in the water... can you buy some distilled water?

check this company out. They make a grout stain in hundreds of colors . It casn be used to recolor any grout. They have a good technical department you can call.
www.goutdye.com

thanks for the replies, I will try distilled water in a small bowl, but if that is the only solution it won't be sustainable as the bowls I am talking about hold roughly 60 litres of water, I really need to just fill them up with the hose.
Thanks for the web site I had a quick look lots of interesting products.
I might just have to use grey grout:(
Thanks again Deb.

You shouldn't have to fill it with distilled water, just make your grout with it

This might help. I use the Mapei brand also, but when I grout outside things, I use the mortar for grout. You can color it w/pigment or Tints All.