Litunin á hárinu hennar endist lengur en hún hélt.

Litunin is the definite form of litun, which means dyeing, coloring, tinting, or the color treatment depending on context.

• litun = coloring / dyeing
• litunin = the coloring / the dye job

The ending -in is the feminine singular definite ending here. Icelandic often adds the equivalent of the to the end of the noun instead of using a separate word.

So litunin literally means the coloring.

Because á here means on/in in a location sense, and that requires the dative case.

So:

• hár = hair
• hárið = the hair (nominative/accusative)
• hárinu = the hair (dative)

After á:

• movement onto something often takes accusative
• location on/in something takes dative

Here the meaning is location/state: the coloring is on/in her hair, not moving onto it. So Icelandic uses:

• á hárinu hennar = on/in her hair

If you said hárið hennar, that would mean her hair as a noun phrase by itself, not on/in her hair.

In Icelandic, possessives like hennar often come after the noun, especially in ordinary phrases like this.

So:

• hárið hennar = her hair
• á hárinu hennar = on/in her hair

This word order is very normal and natural in Icelandic. English learners often expect the possessive to come first, but Icelandic usually does not do that with hennar, hans, þeirra, etc.

Yes. Endist comes from endast, which means to last, to endure, or to remain usable/effective.

Compare:

• enda = to end / finish
• endast = to last

So:

• Litunin endist = The coloring lasts

The -st form is very common in Icelandic and often gives a verb a slightly different meaning. Here endast is the standard verb for last.

Because lengur is the comparative form of lengi, which relates to time duration.

• lengi = for a long time
• lengur = longer

Since the sentence is about how long the coloring lasts, lengur is exactly the right word:

• endist lengur = lasts longer

You would not use meira here, because meira means more, not longer.

Yes, halda very often means hold, but it can also mean think, believe, suppose in certain contexts.

Here:

• hélt = past tense of halda
• in this sentence it means thought / believed

So:

• hún hélt = she thought

This is one of those verbs whose meaning depends a lot on context. Icelandic learners commonly notice this because the basic dictionary meaning hold is not what is meant here.

This is a very common kind of omission in Icelandic, and English does something similar.

en hún hélt means:

• than she thought
• more fully: than she thought it would

The rest is left out because it is understood from context. The full idea would be something like:

• Litunin á hárinu hennar endist lengur en hún hélt að hún myndi endast.

But that would sound much heavier and less natural. So Icelandic leaves the repeated idea unstated.

Because this is a shortened comparative clause.

A fuller sentence might be:

• ... en hún hélt að litunin myndi endast.
• ... than she thought that the coloring would last.

But in the actual sentence, the clause is reduced to just:

• en hún hélt = than she thought

Since the rest is omitted, there is no need for að here.

The sentence mixes present and past, which is perfectly normal.

• endist = present tense, lasts
• hélt = past tense, thought

So the meaning is:

• The coloring lasts longer now than she thought before.

This is the same logic as English:

• It lasts longer than she thought.

Sometimes yes, depending on context.

Literally, it is something like:

• the coloring on/in her hair

But in smoother English, you might say:

• her hair color
• the dye in her hair
• her hair dye
• her hair coloring

Which English version sounds best depends on the situation:

• If you mean the process/treatment, hair coloring may fit.
• If you mean the resulting color, hair color may fit.
• If you mean the product/effect of dye, hair dye may fit.

So Icelandic litun can map to a few different English expressions.

Yes, completely normal.

The structure is:

• Litunin á hárinu hennar = subject
• endist = verb
• lengur = adverb/comparative
• en hún hélt = comparative clause

So the sentence is built very naturally:

• The coloring in her hair
  • lasts
    • longer
      • than she thought

Nothing unusual is happening with the basic word order. The main things learners usually need to notice are the dative after á, the -st verb endast, and the shortened clause en hún hélt.