[Chris M]

In your Admin CP there will be a link in the nav bar, under the Style Manager area, called "Replacement Variable Manager"...

Beside your style name there will be a link to "Add New Replacement Variable" - Click it, and add something like:
{usertitlecolour}
Then add the replacement code for it, like:
#FF0000

Now just use a span tag around the usertitle where it is shown (posts and profiles):
Posts:

<span style="color: {usertitlecolour};">$post[usertitle]</span>

Profiles:

<span style="color: {usertitlecolour};">$userinfo[usertitle]</span>

Chris