Just for clarity: When sending a message to twitch, is that message originating from within your web browser or from an mIRC instance?

If from the webbrowser, this is not the site/program to do such.

If from mIRC you can use the following:
Code:
;; when text is input in a channel window
on *:INPUT:#:{
  ;; if ctrl+enter was NOT used
  ;; the text isn't being pasted in
  ;; the text doesn't start with /
  ;; the text doesn't start with a user defined command character
  if (!$ctrlenter && !$inpaste && /* !iswm $1 && $comchar $+ * !iswm $1) {
    
    ;; pick a random 16bit color, then convert it to a 6-character hex value
    var %color = $chr(35) $+ $base($rand(0, 16777215), 10, 16, 6)

    ;; msg the channel with the color change
    msg $chan .color %color
  }
}