Learn Croatian: Ujesen je sve žuto i smeđe. - In the autumn everything is yellow and brown.

Yes, you can! When talking about seasons as adverbs of time, Croatian often merges the preposition and the noun into a single word, like ujesen (in autumn). However, writing it as two words (u jesen) is also completely correct and very common. You will also frequently hear na jesen. All of them mean exactly the same thing.

Why do the color adjectives end in -o (žuto) and -e (smeđe)?

They are describing the pronoun sve (everything), which acts as a neuter singular noun. Because of this, the adjectives must take neuter endings to match. The standard neuter ending is -o (making žuto). However, after a "soft" consonant—like the đ in the root smeđ (brown)—the neuter ending changes to -e, giving us smeđe.

I always mix up sve and svi. Why are we using sve in this sentence?

Sve translates to "everything" (or "all things" in general) and is grammatically neuter singular. Svi translates to "everyone" or "all" (when referring to people or masculine plural nouns). Since autumn changes the color of things in nature rather than people, we use sve.

Why does the verb je come before ? Could I say "Ujesen sve je žuto..."?