Looking at his foal pics I am going to have a stab at his colour. I think he is fading black. The colour of him as a foal, that silvery grey, is a common colour for black horses, not so much for any shade of red.
The reason I think his sire could be brown is the brown in his flank and down his rear. These are indicators of brown rather than bleached black - the inner flank there would be the last place to bleach if it was.
The full brother could be fading black or liver chestnut, those pictures aren't big enough for me to see. Either way, they are all taken on the same day, and pictures over several seasons are usually best for telling a horse's colour. Or foal and adult pictures.