Maybe grout wasn't mixed thoroughly before using? If you had someone do it you should ask them to come back and look and give their opinion. Were trim pieces grouted on same day as other tile, or maybe two different bags of grout were used. If you had a tile guy do this then definitely get him to explain and he can probably fix it without regrouting. Good luck!

To me mottled grout is not acceptable if you're paying for the installation. I agree with lisadlu that you can get mottled grout from improper mixing of the grout. I believe you can also get mottled grout from excessive water usage during cleanup which can saturate some of the grout. One other possibility (I think I've read somewhere) is a problem with the grout itself. Do you know what grout was used (manufacturer, etc)? I would speak with your tile installer ASAP and not pay for the work until the issue is resolved.

If glazed ceramic tiles were used, sometimes, the glaze runs over from the tiles and is absorbed by the grout. Since it is absorbed unevenly, hence, the mottled grout. You can get the contractor to treat it with grout colorant, so that it'll be one uniform color.

There's a color called Bright White Grout Renew (Polyblend #381) on Home Depot, if that is the color you'd like.