I think it's impossible to accurately describe colour to a blind person, or a colour that a colour blind person can't see.
That said, I think it's possible to give a general idea using other senses for a blind person. For example, a bright colour could be likened to a fast high pitched noise like whistle, or a sharp tasting food like a lemon. A dull colour would be likened to a slow low pitched sound like a digeridoo, or a savoury tasting food like potatoe. That'd be for a blanket bright vs dull explanation, any further difference I don't think could be described so well, the best being compare the difference between two seperate sharp tasting foods etc.
For a clour blind person, maybe you could gradually get closer and closer to the colour that they can't see through shading, from either side of the colour spectrum, until they meet and the colour can't be seen. Then say imagine if they'd be able to keep going until they meet, that'd give a fairly accurate imagination of the true colour, maybe? I don't understand how colour blindeness works, it's always kinda confused me, but would that stand a chance or am I talking out my arse?