kono iro ga awanai nara, betu no iro ni simasu.

Because 合う is not usually used like a direct transitive verb here.

In 色が合う / 合わない, the color is treated as the thing that matches, suits, or works, so it takes が.

• この色が合う = this color suits / matches
• この色が合わない = this color doesn’t suit / doesn’t match

Using を would sound unnatural here.

合わない is the negative form of 合う.

合う can mean things like:

• to match
• to suit
• to fit
• to go well together

So in this sentence, この色が合わない means something like:

• this color doesn’t suit
• this color doesn’t match
• this color doesn’t work

Exactly how you translate it depends on context. For example, it could mean the color doesn’t suit a person, an outfit, a room, or some other item.

なら means if, often with the nuance of if that’s the case or if it turns out that....

So:

• この色が合わないなら = if this color doesn’t suit / if this color won’t work

Compared with other conditional forms, なら often sounds a little more like reacting to a situation or assumption.

For example:

• この色が合わないなら、別の色にします。 = If this color doesn’t work, I’ll go with a different color.

Yes, in many situations you could say:

• この色が合わなかったら、別の色にします。

This also means If this color doesn’t suit, I’ll choose a different color.

The nuance is a little different:

• なら: if that is the case / assuming that
• たら: if/when it turns out that

So:

• 合わないなら sounds more like a judgment or condition
• 合わなかったら sounds more like checking the result and then acting

Both are natural, but なら fits well when you are discussing options.

別の means another or different.

• 別 = separate, different
• 別の色 = another color / a different color

The の connects 別 to the noun 色.

So:

• 別の色にします = I’ll make it a different color / I’ll choose a different color

Because 別 by itself is a noun-like word, and の is used to connect it to another noun.

So:

• 別 = separate / another one
• 別の色 = another color

This is similar to patterns like:

• 日本の車 = a Japanese car
• 私の本 = my book
• 別の方法 = another method

Here, 〜にする means to make something into... or more naturally to choose/decide on...

So:

• 別の色にします literally = I will make it into a different color
• natural English = I’ll choose a different color / I’ll go with a different color

This pattern is very common:

• コーヒーにします。 = I’ll have coffee.
• これにします。 = I’ll take this one.
• 青にします。 = I’ll choose blue.

So in your sentence, the speaker is deciding on a different color.

Japanese often leaves the subject unstated when it is clear from context.

So 別の色にします could mean:

• I’ll choose a different color
• we’ll choose a different color
• let’s make it a different color
• sometimes even you should choose a different color, depending on context

Likewise, この色が合わない does not explicitly say for whom or with what the color doesn’t suit. That also comes from context.

This kind of omission is very normal in Japanese.

が is the natural choice because 色 is the thing being evaluated as matching or not matching.

• この色が合わない = this color doesn’t suit

Using は is possible in some contexts, but it changes the nuance. は would add contrast, like:

• この色は合わない = this color, at least, doesn’t suit

That can sound like you are comparing it with other colors.

In a neutral sentence like this, が is more straightforward.

No. 合う is used with many kinds of things.

Examples:

• 服が合う = the clothes suit someone
• 靴が合わない = the shoes don’t fit
• この仕事は自分に合わない = this job doesn’t suit me
• 味が合う = the flavors go well together

So with 色, it means the color matches or suits something.

Yes. The exact English depends on the situation.

It can mean:

• I’ll choose another color
• I’ll make it a different color
• I’ll switch to another color

For example:

• in a store: I’ll choose a different color
• in design or painting: I’ll change it to a different color

Japanese often leaves that distinction to context.

This is normal in Japanese.

In complex sentences, clauses before the end often use plain form, while the final verb shows the overall politeness level.

So:

• 合わないなら = plain form inside the conditional clause
• します = polite sentence ending

This is a very common pattern:

• 時間がないなら、やめます。
• 必要なら、持ってきます。

Only the final verb usually needs to be polite.

No, it is not strictly necessary, but it helps readability.

So both are fine:

• この色が合わないなら、別の色にします。
• この色が合わないなら別の色にします。

Japanese punctuation is often more flexible than English punctuation.

Normally, standard Japanese is written without spaces:

• この色が合わないなら、別の色にします。

Spaces are often added in teaching materials to help beginners see the parts more clearly.

So the spaced version is for learning, but the normal written form does not use those spaces.

Yes, both are possible, though the nuance changes a little.

• 別の色 = another/different color
• 違う色 = a different color
• 他の色 = some other color

In this sentence, 別の色 sounds very natural and neutral.

Examples:

• この色が合わないなら、違う色にします。
• この色が合わないなら、他の色にします。

All are understandable, but 別の色 is a very common choice here.

It could be used in many everyday situations, such as:

• choosing clothes
• picking paint colors
• deciding on a product color
• matching furniture or decorations
• selecting a design option

For example:

• If this color doesn’t suit you, I’ll choose a different one.
• If this color doesn’t match the room, we’ll go with another color.

Japanese leaves those details unstated unless they are important.