To remove color from something; to make colorless (British English spelling).
From de- (meaning 'remove') + colour (British spelling of color from Latin color). This is the base verb form in British English, with -our spelling reflecting Norman French influence on British spelling conventions.
British English kept the French spelling 'colour' (like French 'couleur') while American English simplified it to 'color'—this is why you see 'decolour' vs 'decolor,' a spelling difference that traces back to the Norman Conquest of 1066!
Complete word intelligence in one call. Free tier — 50 lookups/day.