Are those spots that don't drain well or completely? Have you tried a 10% bleach solution (if it works, that indicates that it's organic--mold, mildew, etc.).

it looks like wet patches..

Jen- Do you have any photos that you can post which show the preparation and installation? Also, if you know the products that were used, that'd be helpful.

Marble tiles in wet areas always equals unhappy customers. Agree possible slope but don't have full pic. Drain area will always look wetter as it is last spot to dry. Other areas can be under shower head and under corner shelves.

The tiles are not turning colors they are staying wet for some reason. Let the shower dry out for a day or two and see if they dry out. Marble looks great but it is one of the worst choices in a wet area that is used daily.

We have larger hex marble tiles I installed myself and you can see where someone got out of the shower or reached for the towel if too wet for about 1/2 a day.

Try sealing again when dry but as long as there are no underlying issues get used to it. If they dry out without using for a day or two it is most likely fine.

Same thing happened when we installed new shower. Your tiles were not grouted or sealed appropriately. Water is getting underneath the tiles thus the wet spots!! You need to let them dry completely, then regrout and reseal. Our contractor did that and we never had another issue