Normally (on TBT 2.0/before the 3.0 site upgrade), you'd go into your collectibles inventory, click "Edit" (or something of the sort) on the "User Title Color Change" item and then you'd input your hex code exactly as it is and check the live preview to see if it worked. The item is single-use only, so if you input it wrong and forget to check said preview it'll have gone to waste.
I don't think you can use it just yet though, as everyone who bought/owned said item before the site upgrade has had it hidden/disabled by staff since there was an issue regarding it (and a few other things) last night when the shop/collectibles system was brought back to the site.