Actually, for the 10g, provided you have numerous plants (fake are ok, but I would go for real), a sorority of at least 5 but no more than 7 (some might argue that 7 is too many for a 10g, but in my opinion, it's a good # for a sorority in this size tank) is probably a better bet than other species of fish since bettas are so picky. If you go for sorority, purchase at least 4 more at same time (good idea is to buy from a local breeder who can give you same age and size...juveniles or young adults from same spawn are ideal)...remove female you have already purchased, move decor around a bit (so existing female might think she's in New tank...will hopefully make her less territorial!), and make sure you have "cave" for each female and lots of plants for breaking up line of sight and giving additional hiding places. Then you can introduce all females to tank and each other at same, including the one you already have.
It should be ok even if the one you already have is a bit older...if you get more from petstore, chances are you will be getting juveniles anyway (in fact, monitor sorority closely for a while as it is quite common to purchase a young female complete with "egg spot" only to find she has grown some amazingly long and colorful fins...in other words, he was mislabeled as a girl:) and so they will be able to get to know each other before becoming adult and territorial.
If you want to get other fish besides 1 betta, be careful as your betta will have issues with: schooling fish that are too energetic such as danios, too flamboyant such as mollies or guppies, or any other aggressive, territorial fish. Ember, neon, or glowlight tetras are good choices as they have color, are quiet and stick to themselves, and they live in middle area of water column. They are schooling fish and are only truly comfortable with at least 6...so 6 neons, etc.
Another good choice are corydoras catfish, smaller ones such as panda, dwarf, (or my favorite) sterbai. Not very colorful but beautiful patterning. Very playful but since they live on bottom, shouldn't upset betta....schooling fish that work best w/at least 6.
Problem is that you asked about colorful tankmates. Most mates that won't upset your,betta are going to be your quiet schoolers and the minimum # for a school is also going to be maximum for your 10g...1 betta and 6 neons....1betta and 6 corydorus catfish....in other words, don't overstock your tank or it will always be dirty which leads to death.
If it were me: I would build the sorority as you will have more flash and color with 5-7 betta females than with 1 betta and 6 ember or neon tetras!
Biggest newbie mistake is to overstock in a variety of ways. Second biggest mistake is to disregard how a particular species lives: when I first started out, I put in a 20g 2 mollies, 4 guppies, 4 neon tetras, 2 white skirt tetras, 1 kuli loach, 3 green brochis catfish (like a corydorus), and 1 plecostamus(probably spelled wrong) who was not a bristlenose and grew to 14inches in length before dying after 9yrs! After lots of research, I got more tanks and made these poor abused (yes, abuse, since in my book ignorance doesn't give you a free pass!) fish as happy as I could make them.
So, forgive me if I sound short or mean...I just don't want anyone making same mistakes! Use us on this forum, we're here to help! :)