vehicle chat is yellow, group chat is purple...
You can put it into a nice little script like here's one of my samples from my mission, and its called from a trigger when a unit leaves the little area...
spetz sidechat "Urgent news Dom, I can't link up with you at the old outpost. It seems some National Guardsmen have decided to take a break there."
~9
pilla SideChat "Okay, okay I'm sure we could go around them right?"
~7
spetz SideChat "No I don't think so, we should probably snuff them to be safe. Plus they have a radio."
~8
pilla SideChat "You better be right, I really don't want to get into a serious scrape so far from the target. We'll meet you there."
~8
spetz SideChat "Okay I'll take up an attack position, see you there."
exit
Of course you can mod the names, delays, and what they say too...
Also if you depbo a few BIS missions you will find that they use gamelogics with waypoints and delays to do radio or converstaions. If you wanna do that, there is a Gamelogic tut in the ed depot.