Ważny jest nie tyle kolor, co frakcja.

This is a very common Polish contrast pattern. It means:

not so much X as Y
or
less X than Y; rather Y than X

So Ważny jest nie tyle kolor, co frakcja does not mean a simple not X but Y. It usually suggests that X is not the main point, while Y is the more important thing.

So the nuance is:

• kolor is not being completely ruled out
• frakcja is being presented as the more relevant factor

Yes, co often means what, but not here.

In this sentence, co is part of the fixed expression nie tyle ..., co .... In that pattern, co works more like as, but rather, or than in English contrastive phrasing.

So here:

• nie tyle kolor, co frakcja = not so much color as faction/fraction

This is a set structure, so it is best learned as a whole expression.

Because the adjective is agreeing grammatically with kolor, which is masculine singular.

• kolor → masculine singular
• ważny → masculine singular

Even though frakcja is the item that gets the stronger emphasis semantically, the sentence is structured around a contrast where kolor is the grammatical anchor of the statement.

So:

• ważny kolor = important color
• therefore Ważny jest ... kolor ...

This kind of agreement can feel a little unintuitive to English speakers, because the real point of the sentence is frakcja, but the grammar still follows kolor here.

Because they are the contrasted nouns in the subject position of the sentence.

The basic idea underneath the sentence is something like:

• Kolor jest ważny = The color is important

Then that gets modified into a contrast:

• not so much color as faction/fraction

Since both nouns are functioning as the alternatives being contrasted in that subject-like role, they appear in the nominative:

• kolor
• frakcja

Polish word order is much freer than English word order. Starting with Ważny jest ... puts emphasis on the idea of importance right away.

So this order sounds a bit like:

• What matters is not so much the color as the fraction/faction
• Important is not so much the color as the fraction/faction

That second version is not natural English, but it shows the structure more closely.

In Polish, putting the adjective first is a normal way to shape emphasis and information flow.

Not necessarily.

That is an important nuance. The construction usually means:

• color is not the main issue
• fraction/faction matters more

So it often downplays the first element rather than completely rejecting it.

If the speaker wanted a stronger, sharper contrast, Polish might use a different structure closer to not X, but Y.

Sometimes, but not always.

Polish frakcja can mean different things depending on context, for example:

• faction in politics or groups
• fraction in chemistry, physics, or other technical contexts

So you should not assume a single English equivalent without context. The grammar of the sentence stays the same either way, but the best translation of frakcja depends on the topic being discussed.

Not very naturally.

A word-for-word rendering would be something like:

• Important is not so much color as fraction/faction

That helps show the structure, but it is not good natural English.

A more natural English version would usually be:

• What matters is not so much the color as the fraction/faction.
• The important thing is not so much the color as the fraction/faction.

So for learning purposes, it is useful to understand both:

• the literal structure
• the natural English equivalent