Mi hermana quiere una mecha más clara, pero yo prefiero el color de su pelo así.

In this sentence, mecha is a hair word. In Spain, una mecha usually means a streak/highlight of hair.

A very common salon phrase is hacerse mechas, meaning to get highlights.

Be careful: mecha can also mean wick in other contexts, but here it clearly refers to hair.

Because mecha is a feminine singular noun, and the adjective has to agree with it.

• una mecha → feminine singular
• clara → feminine singular

So:

• una mecha más clara = a lighter streak/highlight

If the noun were masculine, you would use claro instead.

Not in the usual English sense of clear. With colours and hair, claro/a usually means light.

So:

• pelo claro = light hair
• más clara = lighter

This is a very common false-friend area for English speakers, because claro does not always translate as clear.

Yes. Spanish often leaves the second part of the comparison understood from context.

Here, una mecha más clara means something like:

• a lighter highlight
• a highlight lighter than the current one / lighter than her hair now

You do not need to say the full comparison if it is obvious from the situation.

Yes, Spanish could simply say pero prefiero. The subject pronoun is often omitted.

Here, yo is included for contrast or emphasis:

• Mi hermana quiere... pero yo prefiero...

So the sentence is highlighting the difference between my sister's preference and my preference.

Su can mean several things in Spanish, depending on context:

• his
• her
• your (formal singular)
• their

In this sentence, context tells us that su pelo means her hair — the sister's hair.

This is very normal in Spanish: the possessive itself is sometimes ambiguous, and the context clears it up.

Both are possible, but they have a slightly different feel.

• pelo = the everyday, normal word for hair
• cabello = a bit more formal or literary

In casual speech in Spain, pelo is extremely common and natural. So el color de su pelo sounds very normal.

Because el color de su pelo is the natural Spanish way to say the colour of her hair / her hair colour.

Spanish often uses de structures where English might use a noun as an adjective.

So:

• el color de su pelo = the colour of her hair

By contrast, su color de pelo sounds much less natural here.

Here así means something like:

• like this
• like that
• the way it is

So prefiero el color de su pelo así means the speaker prefers her hair colour in its current form / the way it looks now.

Putting así at the end is very natural in Spanish.

Literally, una mecha is one streak/highlight.

That said, in real-life hair talk, people very often use the plural mechas when talking about highlights in general:

• Quiere mechas más claras = She wants lighter highlights

Using the singular here is still fine if the speaker is referring to one specific streak or just speaking in a more concrete way.

Because así is describing the way the hair colour looks or is left right now.

Spanish commonly puts así after the thing being described:

• Me gusta así
• Lo prefiero así
• Prefiero el color de su pelo así

It sounds natural because the idea is I prefer it this way.

No. Preferir usually takes a direct object, and here that object is already expressed:

• prefiero el color de su pelo así

You do not need an extra pronoun like le here. In fact, le prefiero... would sound wrong in this sentence.

A shorter version would be:

• Lo prefiero así = I prefer it like this

where lo stands for the thing being preferred.