watasi ha zimina iro no huku ga ooi keredo, tiisana moyou ga aru syatu ha suki da.

Because 地味 is a na-adjective.

When a na-adjective directly modifies a noun, it takes な:

• 地味な色 = plain/sober colors

Compare:

• 地味だ = is plain
• 地味な服 = plain clothes

So 地味な色 is correct, not 地味い色.

地味 means something like plain, subdued, quiet, or not flashy.

In this sentence, 地味な色 suggests colors that are not bright or showy, such as muted or conservative colors. It has a nuance stronger than just simple; it often contrasts with flashy, eye-catching styles.

This is a noun phrase built step by step:

• 地味な色 = plain colors
• 地味な色の服 = clothes of plain colors / clothes in plain colors

The particle の links 色 and 服, showing that the clothes are characterized by those colors.

So the whole chunk means clothes in plain colors.

Because 多い does not work like a normal transitive verb.

多い is an i-adjective, and the thing that is numerous or abundant is usually marked with が:

• 人が多い = there are many people
• 仕事が多い = there is a lot of work
• 服が多い = there are many clothes / I have a lot of clothes

So 服が多い is the natural pattern.

Literally, it means the clothes are many. In natural English, it usually means something like:

• I have a lot of clothes
• A lot of my clothes are ...
• I tend to wear a lot of ... clothes

In this sentence, because it is followed by 地味な色の, the meaning is basically:

I have a lot of clothes in plain colors
or
A lot of my clothes are in plain colors

They are doing different jobs.

• 私 は sets the overall topic: as for me
• 服 が 多い marks what is many within that topic

So the structure is roughly:

• As for me, plain-colored clothes are many

This is a very common Japanese pattern: one noun is the topic with は, and another noun inside the statement is marked with が.

けれど means but, though, or although.

It connects the two ideas:

• I have a lot of plain-colored clothes,
• but I like shirts with small patterns.

It sounds a little softer and more conversational than a very strong but in English. It often introduces a contrast without sounding too blunt.

This is a relative clause, which is very common in Japanese.

The part before シャツ describes the shirt:

• 小さな模様がある = has small patterns
• 小さな模様があるシャツ = a shirt that has small patterns

Japanese puts the descriptive clause before the noun:

• [small patterns exist] shirt
• in natural English: a shirt with small patterns

There is no word like that or which required.

Both can appear in Japanese, but here 模様があるシャツ is the straightforward pattern.

• 模様がある = there are patterns / it has patterns
• 模様があるシャツ = a shirt that has patterns

You may also see 模様のあるシャツ, which is similar and often a bit more tightly noun-linked. In this sentence, 模様があるシャツ is perfectly natural.

Both are possible.

• 小さい模様
• 小さな模様

Both mean small pattern(s), but 小さな is a special pre-noun form that often sounds a bit more literary, descriptive, or fixed as a modifier.

In everyday Japanese, both can work here. The sentence simply uses 小さな.

This は marks シャツ as the topic or contrastive topic of the second clause.

So the contrast is something like:

• I have a lot of plain-colored clothes,
• but as for shirts with small patterns, I like them

The は helps emphasize the contrast with what came before.

In modern Japanese, 好き is usually treated like a na-adjective / adjectival noun, not like a regular verb.

So Japanese says:

• Xが好きだ = I like X

Literally, it is closer to:

• X is liked / pleasing

That is why it does not behave like an English verb.

In this sentence:

• 小さな模様があるシャツは好きだ

Because シャツ is marked with は for topic/contrast, the underlying idea is still:

• シャツが好きだ = I like shirts

Because Japanese often omits things that are already understood from context.

The sentence begins with 私 は, so in the second half, I is still understood:

• (私は) 小さな模様があるシャツは好きだ

Repeating 私は would be possible, but it is unnecessary here.

だ is the plain-form copula.

Since 好き behaves like a na-adjective / adjectival noun, it can end with:

• 好きだ = plain style
• 好きです = polite style

So this sentence is in a plain, informal style. If you wanted to make it polite, you could say:

私は地味な色の服が多いけれど、小さな模様があるシャツは好きです。