When using the CC/Custom Colours version AND you have Code Editor enabled, you:

1. highlight the accent colour and press Control/Command + F.  If done correctly, a white box appears in the top right.
2. Click the + button at the bottom and another text entry bar pop ups below it.
3. enter your new colour and then select 'All'.  After that, all of the the selected colour codes should be changed!

you can also read this tip from avicode (link)!!  they both do the same thing but just incase you need another explanation! <3