World Of Colour

[canlock]

Usually unimpressed by Baldric’s cunning plan -11. Not the spelling of Baldric.
State of mind near the end of the spectrum, wrote the duke -4,6. Something to do with violet or purple.

[Buenchico]

The person who was never impressed by Baldric's plans was Blackadder - but that's only 10 letters?

[bobbinwales]

Deep purple. Just a guess

[Buenchico]

Not a bad guess, in my opinion, Bob ;-)

[Captain2]

2 Mood Indigo - Duke Ellington hit

[Captain2]

... ie 'state of mind' = a mood

[canlock]

But BALDRIC is spelt without a K. A Baldric is a leather belt that holds a sword.

[Etch]

I would suggest that the puzzle compiler, like Buenchico, thinks that the name of the Blackadder character is spelt without a 'k'. The word is capitalised, so clearly a name. They've also got the letter count for the answer wrong - maybe you're supposed to append the missing 'k' to the answer, making it 'Blackadderk'. ;-)