In-Post text coloring?

Do you mean like highlighting? or an entire paragraph? or the whole post?

Well you are going to need to disable the WP html parser. Then define some spans in css. Then put in the relevant span in the paragraph.

I mean like a simple html interface when posting, so that you don’t have to roll your own html. Should be able to support changing colors to a user-defined list (or at least a config-based list)
ideal operation would be just like the [em] button and then just color whatever text I need to that color in the post.

Style in your CSS file:
em { color:#ff0099; }
You can style any of the text elements, , , <i>, <b>, etc.

Use p classes
p.red {
color: #ff0000;
}
p.green {
color: #00ff00;
}
p.blue {
color: #0000ff;
}
When posting:
<p class="red">This text would be in red
<p class="green">This text would be green
Not ideal, and colour changes cannot be on the same line, but it works.

Do you mean you’re looking for a way to edit the admin area so you can add tags with a button like in phpBB?