The reddest is the correct one

well either could be correct, depending on the circumstances. So several cars could be a reddish colour, but some are paler or a more orangey shade of red. Reddest car means the car that has the quality of being red to the greatest degree. The most red car wouldn't be used in the above scenario, but could be used where there were several cars that had some red on them, but were not only red - perhaps they had red and white. The most red car would be the one with the greatest proportion of red compared to white. - although on reflection, in that situation, I would probably say 'the care WITH the most red". Hope that helps!