So, I only know of two colors lol and if you have them appearing on the same side, you need to place your characters on opposite sides of the screen. So, here’s an example:
set format phonetext
EXT. GREEN - DAY
@GIRL stands screen left AND BOY stands screen right
GIRL
Hey.
BOY
Hello.
GIRL
Talk to you soon.
BOY
OK, bye.
…and bla bla bla, more text messages go here until you set format back to cinematic.
Also, since GIRL is standing on the left, her messages will be on the left and they’ll be in white. And since BOY is standing on the right, his messages will be on the right and they will be in blue : )
P.S BOY and GIRL are just example names I used, of course your characters may have different names ^^