Fairly easy if you're decent at mIRC scripting. If you're new to scripting, it's a little more difficult.
That said, you CAN control font/color in EDIT and COMBO controls from mIRC using RTF. Note in /help /dialog, you'll see both controls have "rich" as a property. For details on how to set it up, check out
this post, which also includes a tutorial on RTF if you need to learn what to include in the formatting.
However, keep in mind that this does not work in 7.x. It will only be useful in 6.35 and older. If you want it to work in 7.x, you'll need DCX or another DLL.