Yeah, Black color indeed absorb sunlight(or more precisely the light) better rather than White that reflect sunlight. that's why she said his armor is better.
Sir Agravain,My Armour...I was about to lend mine, but...We're having a grilled boar that our king hunt for lunch.How delicious.T-That's...I'll let it slide...The meat is well-cooked to the inside. It seems that it's cooked with iron plate...(Because it's black.)Heeeeeey!How delicious.Let's grill the next one!She said that "Agravain's armour is the best for grilling"...*munch*