Learn Hardcore Finnish: Keväällä kukka on keltainen, mutta syksyllä se on melkein oranssi. - In spring the flower is yellow, but in autumn it is almost orange.

Using kevässä or syksyssä with -ssa / -ssä (inessive, “in(side)”) would sound wrong here. -ssa is normally used for being physically in something (for example talossa = in the house), not for seasons.

Could I repeat the noun and say … mutta syksyllä kukka on melkein oranssi instead of … mutta syksyllä se on melkein oranssi? Why is se used?

Yes, you can say:

Keväällä kukka on keltainen, mutta syksyllä kukka on melkein oranssi.

That is grammatically correct. However, Finnish, like English, often avoids repeating the same noun when it’s clear from context. So a pronoun is very natural:

Keväällä kukka on keltainen, mutta syksyllä se on melkein oranssi.
In spring the flower is yellow, but in autumn it is almost orange.

Using se simply replaces the repeated kukka and makes the sentence sound more natural and less heavy.

What you normally cannot do is drop the subject completely in the second clause and say:

✗ … mutta syksyllä on melkein oranssi.

That sounds wrong, because on without a subject is usually read as an existential construction (there is…), not as it is…. So you need either kukka again or the pronoun se.

Why is the pronoun se used for “it”? When do you use se and when hän?

In standard Finnish:

se = it / that
hän = he / she (a person)

For things, animals, and inanimate objects, you normally use se, not hän:

Kukka on keltainen. Se on kaunis.
The flower is yellow. It is beautiful.

For people in formal or written language, you use hän:

Hän on opettaja. – He/She is a teacher.

In everyday spoken Finnish, people very often use se even for people, but in writing and in textbooks you should learn:

se → it / that (and sometimes he/she in colloquial speech)
hän → he / she (person, more formal/careful style)

In your sentence the subject is a flower (a thing), so se is the normal pronoun.

Why is there no “a” or “the” before kukka? How do you know if it means “a flower” or “the flower”?

Finnish does not have articles (no words like a, an, the). The bare noun kukka can correspond to:

a flower
the flower
sometimes even flowers in a generic sense, depending on context.

So:

Kukka on keltainen.
can mean A flower is yellow or The flower is yellow.

Which one is intended is decided by context and what has been mentioned before. Here, because we are talking about a specific flower whose color changes between spring and autumn, an English translation with the flower is the most natural.

Finnish does not grammatically mark definiteness the way English does; it relies on context, pronouns, and word order instead of articles.

Why are the color words keltainen and oranssi in their basic form? Why not something like keltaista or oranssia?

After the verb olla (to be), adjectives that describe what the subject is (predicative adjectives) normally appear in the nominative case, matching the subject:

kukka (nominative singular) → keltainen (nominative singular)
Kukka on keltainen. – The flower is yellow.

This is the default when you’re talking about a whole, countable thing and a simple property it has.

You sometimes see forms like kylmää, kuumaa, pitkää (partitive) after on, for example:

Vesi on kylmää. – The water is cold.
Maito on kuumaa. – The milk is hot.

That partitive style is more typical with mass nouns (water, milk, etc.) or to express a more “indefinite” or “somewhat” quality. With a single, discrete flower, the normal, neutral choice is the nominative:

Kukka on keltainen.
Se on melkein oranssi.

So keltainen and oranssi are in their basic nominative forms because they are simple predicative adjectives agreeing with kukka / se.

Can I change the word order and say Kukka on keltainen keväällä, mutta melkein oranssi syksyllä? Does that sound natural?

Yes, that word order is grammatically correct and can be natural:

Keväällä kukka on keltainen, mutta syksyllä se on melkein oranssi.
Kukka on keväällä keltainen, mutta syksyllä melkein oranssi.

Both are fine. The difference is mainly emphasis and information structure:

Starting with Keväällä and syksyllä puts more focus on time and the contrast between the seasons.
Starting with Kukka puts more focus on the flower itself, with the times added as extra information.

In everyday speech, putting the time expression first is very common when you want to set the time frame: Keväällä… mutta syksyllä…

Why is melkein before oranssi? Could I say se on oranssi melkein instead?

Melkein means almost / nearly. In Finnish, adverbs that describe degree usually come right before the word they modify:

melkein oranssi – almost orange
todella kaunis – really beautiful

So in:

se on melkein oranssi

melkein modifies oranssi, and this is the normal, natural word order.

Putting it after the adjective:

✗ se on oranssi melkein

sounds wrong or at least very unnatural in standard Finnish. The default rule for this kind of adverb is:

[verb] + [degree adverb] + [adjective]
on melkein oranssi, on todella kaunis, etc.

Why is there a comma before mutta?

In Finnish punctuation, when mutta (but) connects two independent clauses (each with its own verb), you normally put a comma before mutta:

Keväällä kukka on keltainen, mutta syksyllä se on melkein oranssi.

Both parts have their own verb on, so they are two clauses joined by mutta. Hence the comma.

You usually do not use a comma with mutta if it’s just joining two adjectives or other small phrases instead of full clauses:

hyvä mutta kallis – good but expensive (no verb, so no comma)

In your sentence, each side is a full clause, so the comma is required in standard written Finnish.