If you're talking about bright white text, I *think* that indicates a private transmission. For example, to send me a message on a private channel, you'd type ".p oboe " followed by your message, e.g. ".p oboe Hi oboe, nice to see you in game"
This message would show up to me in all white text. Channel 200 is more of a grey or dull white text.
And to respond to a private chat message someone else initiated, you type ".r " followed by the your message (leave a space between ".r" and your message)