This won't have the color picker, but alternatively you can utilize mIRC's ability of having a second editbox. In your treebar, or active icon from the menubar, or if you still use the switchbar (left click for the menubar if you're clicking the active icon, right click for treebar/switchbar) and in there you'll see a menu option as:

"Edit Box..."

If you enable this (and set it as default) You can pre-set a command within the Command field, such as /me

This will give you an alternative editbox to use which would have /me pre-loaded for you. I'm sure with a bit of tinkering and an ON *:TAB event you could have a cycling color code working on that editbox too. This would save you time, not require a specialized hot-key, just a simple shift+tab to jump to the 2nd editbox, or a quick mouse over and click into the 2nd editbox, and whatever you chose to type within that box will use whatever command you have set for it.

Unfortunately sending ^k using com-objects to access SendKeys does not prompt the built in color picker, or I thought about that being an option to automatically pop-up mIRC's internal color chooser as soon as you focused over to the other editbox frown