Basically, I write on white things.
There may be a little confusion here, and I mean the original sentences. The translation needs to be corrected. Mixing "ami" with "amelyik" is an unfortunate mistake in Hungarian usage. Hopefully it will not be taught here.
"Arra írok, ami fehér." means "I write on what is white".
That is, I only write on white things.
"Arra írok, amelyik fehér" means "I write on the one that is white".
That is, I pick the white one, out of all the available colors, and I write on that one.
You are welcome! But be aware, colloquial usage may not folllow the "rules". This very sentence above ("Arra írok, ami fehér") means to say "amelyik" but uses "ami" instead. It is okay in some cases, while in other cases may be plain wrong. Here, for example, it can lead to misunderstanding.
Anyway, if you understand the difference, that is a great help in any case, both understanding the correct usage and hopefully being able to decipher the not so correct, lazy usage. You will see a lot of those in this course. Good luck!
And here is a long discussion for you, where I tried to explain all this confusing topic in detail: