means to dye something blood-red or for it to turn red (in the case of Macbeth)...
However, before Macbeth it originally meant a pale pink colour (actually, it meant flesh coloured, but 'tis close enough) and 'twas used as an adjective. In Macbeth it is used as a verb, in fact, it's the first time for it to be used in this way.
“Will all great Neptune’s ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red”.
