There is a five-tier bookshelf next to the desk and many books, including my English books, are there.

On the left side is my first son's desk, which color isis sky bluesky and has a cute shape.

So, first: you can just use a colour as an adjective. It is red, it is blue. You don't have to specify that you're talking about colour. That's simplest and what I suggest.

Secondly: this is probably confusing, but "which colour" is wrong here. The correct word, if you want to talk about colour, would be "whose": "A desk whose colour is sky blue." We use "whose" as a relative pronoun to say that something *belongs* to something else. Usually that's about people, but sometimes it's about other things, like attributes of inanimate objects: the desk's colour, the colour of the desk. Or the tree's leaves, the car's wheels, things like that. A car whose wheels are chrome, a tree whose leaves have fallen.

Sometimes you can say "which has": "A car which has chrome wheels" (or "which ... has a cute shape"!) But sometimes you need to reword the sentence to use that: "A tree which has lost its leaves", and sometimes the rewording would be very awkward: "A desk which has blue colouring."

Next to my son's onethat, there are two display cupboards with a lot of Lego figures and toys

I can't figure out a satisfying explanation for this, but it feels wrong. I think you'd be more likely to say "one" like this if there were multiple desks in the same sentence? "There are two desks, mine and my son's, and next to my son's one there are two cupboards."

My favorite piece of furniture is the cabinet on the wall, because it has my treasures.

I don't think what you have is too bad, but we usually use something like "piece of" to make it clear we mean a single item of furniture.

Using cameras, I am able to take pictures of my memories.

You're being poetic, right? I think that's fine :) but *strictly* interpreted this would be incorrect, your memories are in your head and it's rather difficult to take pictures of them.

And I write memories with my fountain pens.

Similar story here. You could write down your memories or write about your memories, you wouldn't simply write memories. But as poetry I rather like it.

So, I used to go to my study room and enjoy my memories.

used to - you don't any more? Do you mean something like "I'm accustomed to"? That might be "I am used to going to my study room and enjoying my memories."