Ctrl-K followed by a number 0-15 will change your color. Then, just type text. To turn the color off, either change it to something else the same way, or use Ctrl-O to remove all color and control codes.
For a message, use a timer (note that a message every 25 seconds will probably get annoying, but it's up to you and the ops in the channel).
/timerAutoMsg 0 25 msg #yourchannel This is my auto-message. Enjoy.
Replace #yourchannel with the name of your channel. 25 is the number of seconds between messages. 0 means to continue forever (or until you close mIRC).