iro ga awanai node, henpinshitai desu.

to be

Breakdown of iro ga awanai node, henpinshitai desu.

• が: subject particle
• 〜たい: to want to
• ので: reason particle

Questions & Answers about iro ga awanai node, henpinshitai desu.

Why is が used after 色 instead of は?

が marks what doesn’t match as the specific subject/new information inside the reason clause. 色が合わないので focuses on “the color (specifically) doesn’t match, so…”. You could use は to topicalize or contrast: 色は合わないので… implies “as for the color, it doesn’t match (though other aspects might be fine).” Both are grammatical; が is the neutral choice here.

What exactly does 合わない mean here, and how is it different from 会わない?

Here 合わない is the negative of 合う “to match/go well/fit (in terms of compatibility).” It means “doesn’t match.”
Be careful: 会う (same pronunciation) means “to meet.” Here it’s 合う (match), not 会う (meet).

Should I use 合う or 似合う when talking about color?

• Use 合う for things matching with other things: このシャツの色がズボンに合わない (The shirt’s color doesn’t go with the pants).
• Use 似合う when something suits a person: この色は私に似合わない (This color doesn’t suit me).
  If you mean “it doesn’t suit me,” prefer 似合わない.

Do I need to say what it doesn’t match with? Where would に go?

You can specify the counterpart with に:

• このスカートの色がトップスに合わない。