All this talk of Betta females has me considering a buddy or 2 for my lone
male Betta. He seems very peaceful, as I've never seen him flare. But he's
only in a 10 gallon tank, so I'm concerned that it would not be enough
space for the three of them.

Should I put a divider in the tank and see how he reacts to the two
females before letting them interact? He's been in the tank about six
months now, so he likely considers it his personal space. Should I add
both females at the same time or space them out? Any preference for the
colours of the females (male is Indigo with red highlights) in terms of
compatibility?

I have had good luck with a similar approach: a 10-gallon tank with a mesh
divider, a male betta on one side, and a female on the other. At first he
may flare at her a lot and she may run and hide, but after a while (a few
weeks) you may find that they spend all of their time hanging out "together"
at the divider. After a few weeks or a month of this, remove the divider and
see what happens. In most cases, they get along just fine and often stay
together most of the time but usually wouldn't breed. I did note that they
tended to sleep apart, in their own original territories. All in all, just
like an old married couple

I don't think colors matter but I always tried to match them up (blue (or at
least bluish) females with blue males, red females with red males, and so
on).

--
John Goulden
mostly guppies, goldies, swordtails, and bettas