Stoffprøven er ikke mørkere, men tvert imot lysere.

Stoffprøven is the definite singular form, meaning the fabric sample or the material sample.

• en stoffprøve = a fabric sample
• stoffprøven = the fabric sample

In Norwegian, the definite article is often attached to the end of the noun:

• en prøve = a sample
• prøven = the sample

Here, stoff + prøven gives stoffprøven.

Stoffprøve is a compound noun:

• stoff = fabric, material, or cloth
• prøve = sample or test piece

So stoffprøve means something like fabric sample, material sample, or swatch, depending on context.

In a normal Norwegian main clause, the finite verb usually comes in second position. Negation such as ikke normally comes after that verb.

So:

• Stoffprøven er ikke mørkere
  = The fabric sample is not darker

This is standard Norwegian word order.

Compare:

• Han kommer ikke. = He is not coming.
• Det er ikke sant. = That is not true.

English and Norwegian differ here, because English often places not directly after a helper verb, while Norwegian generally puts ikke after the finite verb in main clauses.

They are the comparative forms of the adjectives:

• mørk = dark
• mørkere = darker
• lys = light
• lysere = lighter

The sentence is making a comparison, so Norwegian uses the comparative form, just like English does.

Many common adjectives form the comparative by adding -ere:

• stor → større = big → bigger
• fin → finere = nice → nicer
• mørk → mørkere = dark → darker
• lys → lysere = light → lighter

But not all adjectives behave exactly the same way, and some are irregular.

Also note that the sentence only uses the comparative, not the full comparison with enn. It does not say what it is darker or lighter than; that comparison is understood from context.

Men tvert imot means but on the contrary or quite the opposite.

It strongly corrects or reverses the first idea.

So:

• ikke mørkere, men tvert imot lysere = not darker, but on the contrary lighter

This is stronger than just men lysere. It emphasizes that the opposite is true.

Yes. Stoffprøven er ikke mørkere, men lysere is grammatical and natural.

Adding tvert imot gives extra emphasis:

• men lysere = but lighter
• men tvert imot lysere = but quite the opposite: lighter

So tvert imot is not required, but it strengthens the contrast.

Enn means than and is used when the second part of a comparison is stated explicitly.

For example:

• Stoffprøven er lysere enn den andre.
  = The fabric sample is lighter than the other one.

In your sentence, the comparison point is left unstated because it is already understood from context. The sentence only says that it is not darker, but rather lighter.

Yes. In this context:

• mørk = dark
• lys = light

So mørkere and lysere are natural opposites when talking about color or shade.

Be aware that lys can also mean light in other senses, and stoff can mean different kinds of material in different contexts, but here the color meaning is the important one.

Yes. Stoff is a flexible word in Norwegian. It can mean:

• fabric / cloth
• material
• substance
• sometimes even subject matter in certain contexts

In stoffprøve, the most likely meaning is fabric sample or material sample. Context tells you which is best.

A simple learner-friendly approximation is:

STOFF-prø-ven er IK-ke MØRK-er-e, men TVERT i-MOT LYS-er-e

A few pronunciation notes:

• ø in prøven and mørkere is a vowel English does not have exactly.
• y is not in this sentence, but lysere has y-like spelling? No — here it is ly, where y is the Norwegian vowel y, also unfamiliar to English speakers.
• tvert imot is often spoken smoothly, almost like one chunk.

If you want, the most important rhythm is the contrast: ikke mørkere, men tvert imot lysere

Yes. The pattern is very natural:

• ikke X, men Y = not X, but Y
• ikke X, men tvert imot Y = not X, but quite the opposite: Y

You can use the same structure in many situations:

• Han er ikke sint, men bare sliten.
  = He is not angry, just tired.
• Det er ikke vanskelig, men tvert imot ganske lett.
  = It is not difficult; on the contrary, quite easy.

So this sentence is a useful model for expressing correction or contrast in Norwegian.