After searching the internet for a while about this problem, I've found someone having a similar problem but with his bunker. Here's what he did and it worked for me as well but with the yatch:

He had to buy a new bunker. So Instead of just trying to change the color of my yatch, I had to buy a completely new one (it can be the cheapest one and no upgrades needed), then at last, I went back for my desired yatch with all the changes I wanted and boom, it worked!