Should the color of grout be as similar as possible to the tile or do you want contrast? we bought our house 5 years after it was built (only one previous owner) and I think the grout is so dark that it must be really dirty....but I can't tell if maybe they chose a dark color instead?

Generally when grout blends in the tiles looks nicer and their detail stands out more (see pictures). If you are not sure if it's dirt or they put in a darker color check in the back of a closet or under a fridge or another appliance. Usually those areas are relatively clean with perhaps just some dust that can be wiped off.

https://www.houzz.com/discussions/what-would-you-do-botched-tile-job-dsvw-vd~1232342

Does this look like polyblend brillant white over versabond white thinset with white penny rounds? My contractor swears it is.

It is hard to tell from the picture especially with the horrible color this monitor has. I would need to see it in person. But don't understand what is the whole discussion about. Pull up a tile and look what thinset was used? If you can't pull it up any handyman can...(The reason he would want to use grey thinset as opposed to white is because if he wanted to grout it right away he could use rapid set thinset which only comes in grey and it dries faster) You may have an alternative to ripping up the floor if its not white. You can get the grout color sealed in bright white and it may possibly work even if some grey thinset is peaking through the grout. Have someone do an estimate for you and do a sample test spot to see results. But give it 24 hours to dry before you evaluate because when it is not fully dry it can appear whiter.