style replacement value
 

i've got a certain colour that i seem to using quite a bit on a styleset and as it falls under none of the pre-defined options i was wondering how i'd go about adding is something like "custom_one" so that if i need to change it i can change it all at once via one screen instead of hunting out all the tables.

In vBulletin 3 there are still replacements, they are just not used by default style vars anymore. If you look at the style opions for that style you can add new replacements that can be used in your templates.