To add UI for font color, it's a bit more complicated. You would look at the js/chatroom.js JavaScript file:
You would modify the create() method near the top of the file to add something like the following (from ajaximrpg 3.41):
' <a href="#" class="setFontColorLink" id="' + winId + '_setFontColor" onclick="Chatroom.windows[\'' + name + '\'].toggleFontColorList();return false;" onselectstart="return false;"><div id="' + winId + '_setFontColorColor" style="width:14px;height:14px;display:block;"></div></a>'
This would be inserted around the line (from ajaximrpg 5.0) that looks like:
html += '<a href="#" class="insertEmoticonLink" id="'+winId+'_insertEmoticon" onclick="Chatroom.windows[\''+winName+'\'].toggleEmoticonList();return false;" onselectstart="return false;"><img src="themes/'+theme+'/emoticons/mini_smile.gif" width="14" height="14" style="border:0;" /></a>\r\n';
You would modify the old line to be similar to the emoticon line because they do sort of the same thing. That should make a button for the font color and allow you to access the original font color popup which you can use as a UI.
The font color pop up box calls the setFontColor() in js/im.basic.js when a font is selected:
You would replace the following code in setFontColor():
sendBox.setStyle({color: color});
with actual HTML like:
sendBox.innerHTML = '<span style="color:green;">'+sendBox.innerHTML+'</span>';
If you were ambitious, you could do text selection detection on sendBox and modify the innerHTML to only change the words that were selected.
Whatever is in the sendBox, the user should be able to press Enter and it will be sent to everybody, including the <span> tags.
That's sort of the broad strokes. You may have some hassle with getting a parent tag instead of the tag itself or that the <span> tag might get stripped out somewhere along the line (like in ajax_act.php).
Let me know if you have more questions, need more help or want more detail.
If you do come up with some code to share, please let me know. I'd like to get the feature working again and I'm willing to integrate it into the main code.