Hi Gill so many questions
Their colours will continue to develop for many weeks to come so don't worry about that - it is fun to see how small patches of white for example seem to grow and spread and creamy areas turn ginger and copper for example...
The empty tumbler test really isnt necessary with Syrians, this is more for sexing Roborovski dwarf hamsters. You can happily pick up Syrian babies and see what sex they are. As it is the first time you have done this see if you can hold two up at once or get someone to help you. Hold both up and look at their genitalia side by side. All those that look the same put together in another cage. You should hopefully end up with two groups...
Okay so you end up with three, some are undecided, that is fine at 21 days old.
Of the two groups that look the same can you now focus on the spacings of the holes and see those where there is a small gap between the anus and front bump
pee hole
Those are your boys. Put the undecided ones in with the boys for now. As girls mature at 4 weeks and boys at 5 you still have plenty time to seperate them out. Now everyday you should be playing with these newly weaned babies, twice if you can manage it. Recheck the genitalia every day and eventually by day 28 you should have only 2 definite groups left hopefully.
Are you removing the babies all at once or leaving one group with mum. Either one complete sex group or the stragglers?
There is no right or wrong decision. You need to do what you feel is best for the babies. If you think some will benfit from some more time with mum, leave them in till they are 28 days old and then move them out to the split sex groups. Don't worry they wont fight even though they have been apart for a few days (well not too much) you will start to see lot of wrestling going on amongst the pups.
Mum will be happy if you remove them all at once don't worry about that, but she will also cope with them in with her for few more days (up to 28 safely).