Jiroh doesn't like red, but he absolutely adores the colours of Gakuto's hair. Normal red is way too bright and he can't sleep well when there are red things around (it's very distracting), but Gakuto's hair is different. It isn't very bright, it's a very dark red, and Jiroh really likes how the red only comes through when it's really light outside.
When he leans on Gakuto's shoulder he is tickled by some of the strands and it feels really nice, and smells good, like cherries. Sometimes it makes Jiroh wonder if he tastes like that, too.
Jiroh likes the red of Gakuto's hair, because it makes him want to kiss the other, and his kisses are very sweet (though they don't taste like cherries at all.)
Gakuto hates his hair. He loves red and he would like to wear it so much, but it would clash horribly with his hair and therefore he can't. The only time he likes it, is when Jiroh leans in and nuzzles his head against his shoulder, because (he only figured that out after a long time) Jiroh likes Gakuto's hair.
So when Jiroh kisses him and mumbles "Still not cherry" afterwards he is really happy his hair is red, because it will keep Jiroh kissing him like that until it's not just his hair that's like cherries.