Welcome, KK.
Please tell us more specifically which MAPEI grout you used. If you have a link, just spell it out with an extra space in the www and a moderator will light it up for you (you'll be able to post links after a couple more posts).
Is this the sealer MAPEI sent you? I know you said sealant, but this is a penetrting sealer, not a sealant. There's a large difference.
1. I'm not sure why you'd wanna do that, but please don't. One, the MAPEI product, is a water based product, while the other, the 511, is a solvent based product, I do believe. I don't know that one would do more good than the other in your situation. Actually, I don't know that either can prevent your washing color out of your grout. Perhaps I'm missing something. What exactly did the MAPEI rep tell you it would do?
2. Either of those products can be used on your entire floor. Whether either will have any meaningful effect on the tiles I cannot tell. Might do nothing but cause a haze if not cleaned off the tile surface properly.
My opinion; worth price charged.