Looking for a Sci-Fi book title:

[Angela5]

Fantasy/Sci-Fi. Kingdom based on colour powers. two brothers fought for the right to be king. One locked the other up in a chamber of blue so he could get no power.

 

[tachyon]

Sounds like one of the books in the Lightbringer series by Brent Weeks possibly. I haven't read these but the magic is based on colors.

 

[tachyon]

Linking this to another query about color magic, but I think they're 2 different books.

I think I remember the magic system, factions, and setting, but not names

I did some more searching for this querent and found this review:

The Black Prism by Brent Weeks book review - Fantasy Book Review

I had never read the previous trilogy and had committed the cardinal sin of buying a book simply because I liked the cover [...]

www.fantasybookreview.co.uk

Which contains a reference to the blue painted prison room:

"And just to spice up the mix, there is the mysterious prisoner that Gavin keeps locked away in a secret blue room..."

So I think my suggestion of Brent Weeks' "Lightbringer" series is correct for this query.