Wir rösten nach, bis die Bohnenfarbe dunkel ist.

Because the verb here is the separable verb nachrösten.

In a main clause, German separable verbs split:

• Wir rösten nach.

The conjugated part rösten goes in the usual verb position, and the prefix nach moves to the end of the clause.

You would see the full verb together in forms like:

• Wir wollen nachrösten.
• ..., weil wir nachrösten.

So rösten nach is not two unrelated words here; it is the split form of nachrösten.

Here, nachrösten means something like:

• to continue roasting
• to roast a bit longer
• to roast further

It does not simply mean to roast after in a literal time-order sense. The prefix nach- often adds the idea of doing something further, afterward, or as a follow-up.

In this context, the sense is: the roasting continues until the beans reach the desired dark color.

Because bis introduces a subordinate clause.

German normally puts a comma before a subordinate clause:

• Wir rösten nach, bis die Bohnenfarbe dunkel ist.

This is very standard German punctuation. English may or may not use a comma in similar sentences, but in German this comma is expected.

Because bis creates a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• main clause: Wir rösten nach
• subordinate clause: bis die Bohnenfarbe dunkel ist

That final ist is a classic sign of subordinate-clause word order in German.

Because die Bohnenfarbe is singular.

A common point of confusion is that die can mean:

• the for a feminine singular noun
• the for a plural noun

Here, Bohnenfarbe is a compound noun whose main word is Farbe:

• die Farbe = the color

So die Bohnenfarbe means the bean color or the color of the beans, and the whole noun is singular. That is why the verb is:

• die Bohnenfarbe ist dunkel

not:

• die Bohnenfarbe sind dunkel

Because dunkel is a predicate adjective, not an adjective directly before a noun.

After verbs like sein, werden, and bleiben, German adjectives usually do not take endings:

• Die Bohnenfarbe ist dunkel.
• Der Kaffee ist heiß.
• Die Maschine bleibt sauber.

You only add adjective endings when the adjective comes before a noun:

• die dunkle Bohnenfarbe

So:

• die Bohnenfarbe ist dunkel = correct
• die Bohnenfarbe ist dunkle = incorrect

It is in the nominative case because it is the subject of the subordinate clause.

In:

• bis die Bohnenfarbe dunkel ist

the subject is die Bohnenfarbe, and ist is the verb. The color is what is dark, so nominative is used.

Yes. German very commonly forms compound nouns by joining words together.

Here:

• Bohnen = beans
• Farbe = color

So Bohnenfarbe literally means bean color or the color of the beans.

This is completely normal German word-building. English often uses separate words, but German often packs the idea into one noun.

Yes. Bis usually means until in this kind of sentence.

So the structure is:

• Wir rösten nach, bis ...
• We keep roasting until ...

That is the basic idea. The German present tense works naturally here for describing a process or procedure.