Hamilton may not be happy with a black one, but also Ferrari wouldn't want a black one neither, as a team. Even if marketing ever suggested it, all other departments will say no to that with ease. Ferrari is red, that's just how it is, and I do not mean about the general car color for formula neither, I am talking about literally regular cars as well.
If you ever bought a Ferrari, would you rather have a black one? I would not consider any other color than red ever, because that's the color of the brand. Other brands could have red cars too, it is not like against the law or anything, but when you see a red Mercedes, or a red BMW, it is not the same feeling as seeing a red Ferrari. Hence, the color is out of question.
Hamilton will definitely need to work harder than before. Because while Mercedes had different cars each year, and it took time to adjust to them all the time, Ferrari is a whole another team together. Obviously Hamilton isn't new to change, just a few years ago, Mercedes had a car that literally jumped up and down all the time, ruining the back of the drivers, I am sure that was even worse than driving Ferrari for the first time.
But the problem is that it's a brand new team, so he will get to drive a lot more to get used to it first. Plus, it is not just the car, the team staff is different too, no more Bono, no more Toto, there is nothing going same for him, he will need to get used to every staff member there instead.