Hvis jeg fik farvet håret rødt, ville min søster sikkert se det med det samme.

Because få + past participle is a very common Danish pattern meaning to get/have something done.

• jeg farvede håret rødt = I dyed my hair red
• jeg fik farvet håret rødt = I got/had my hair dyed red

So fik farvet often suggests that the speaker had it done, for example by a hairdresser, or simply focuses on the result rather than who performed the action.

Yes. That would also be correct.

The difference is mainly nuance:

• Hvis jeg farvede håret rødt = if I dyed my hair red
• Hvis jeg fik farvet håret rødt = if I got my hair dyed red / had my hair dyed red

The version with fik farvet sounds especially natural when talking about changing hair color as a treatment or salon-style action.

Danish often uses the definite form for body parts and personal belongings when the owner is already obvious from the context.

So:

• jeg vasker hænderne = I wash my hands
• jeg brækkede benet = I broke my leg
• jeg fik farvet håret rødt = I got my hair dyed red

Using mit hår is possible, but it would usually add emphasis, contrast, or sound less neutral in this kind of sentence.

Because hår is a neuter noun: et hår.

When Danish uses a color word as a result complement after verbs like farve, the adjective agrees with the noun:

• farve døren grøn
• male huset hvidt
• farve håret rødt

Since håret is neuter, the adjective takes the -t form: rødt.

Because Danish, like English, often uses past forms to express a hypothetical or unreal situation.

So this pattern:

• Hvis jeg fik ... , ville ...

works like English:

• If I got ... , my sister would ...
• If I dyed ... , my sister would ...

This is not real past time. It is modal past: past form used to show unreality, possibility, or imagination.

This is because Danish follows the V2 rule in main clauses: the finite verb must come in the second position.

The first position is taken by the whole if-clause:

• Hvis jeg fik farvet håret rødt

So in the main clause, the finite verb comes next:

• ville min søster sikkert se det med det samme

If there were no fronted if-clause, the word order would be:

• Min søster ville sikkert se det med det samme

Here sikkert is a sentence adverb meaning probably / surely depending on context.

In this sentence, after the V2 inversion, the order is:

• finite verb: ville
• subject: min søster
• sentence adverb: sikkert
• infinitive: se

So:

• ville min søster sikkert se ...

Without the fronted clause, it would be:

• Min søster ville sikkert se ...

It refers to the thing the sister would notice — basically the change, the red hair, or that fact.

Danish often uses det in places where English might say:

• it
• that
• the change
• the difference

So se det here is best understood as notice it.

Yes, lægge mærke til det is a more explicit way to say notice it.

But se det is also very natural in Danish when something is visually obvious. In context, it means something like:

• see it right away
• spot it immediately
• notice it at once

So se det is perfectly idiomatic here.

It means immediately, right away, or at once.

It is a fixed expression, so you should learn it as a whole:

• Jeg kommer med det samme = I’m coming right away
• Hun så det med det samme = She saw/noticed it immediately

Even if the words look literally like with the same, the phrase does not mean that word-for-word in normal use.

Yes, grammatically it is singular: håret = the hair.

But it refers to hair collectively, not one single strand. This is very natural in Danish, just as English uses hair as a mass noun.

So håret rødt means the hair red, not one hair red.

Because hvis is used for a condition:

• Hvis jeg gør det, ... = If I do it, ...

By contrast, om usually means whether/if in indirect questions:

• Jeg ved ikke, om hun kommer = I don’t know whether she is coming

So in this sentence, hvis is the correct word because the meaning is conditional.

Because Danish normally separates a subordinate clause from the main clause with a comma.

Here:

• Hvis jeg fik farvet håret rødt = subordinate if-clause
• ville min søster sikkert se det med det samme = main clause

So the comma marks the boundary between the two clauses.