tomodati ha kabegami no tuya to sasiiro wo mite bakari iru.

は marks 友達 as the topic: as for my/the friend...

In a sentence like this, は is natural if the speaker is talking about what that friend is doing. It gives a descriptive, topical feel.

If you used が, it would sound more like you are specifically identifying the subject, as in the friend is the one who is... That is possible in some contexts, but は is more neutral here.

Also, 友達 can mean friend or friends depending on context.

壁紙のつや means the sheen/gloss of the wallpaper.

Here, の links two nouns. A very common function of の is to show a relationship like:

• possession
• belonging
• attribute
• type/category

So here it is basically wallpaper's sheen or more naturally in English, the sheen of the wallpaper.

つや means gloss, luster, or sheen.

It refers to how shiny or polished something looks. In this sentence, it is talking about the visual finish of the wallpaper.

Depending on context, つや can be used for things like:

• hair shine
• skin glow
• glossy surfaces
• polished materials

差し色 is a design/fashion term meaning an accent color.

It is a color added to a space, outfit, or design to create contrast or visual interest. So in this sentence, the friend is paying attention to things like:

• the wallpaper's sheen
• the accent colors used with it

This is a fairly specific word often used in interior design, clothing, and aesthetics.

と connects nouns and means and.

So:

• 壁紙のつやと差し色 = the wallpaper's sheen and accent colors

This is the normal noun-listing と. It is a straightforward, complete list in this sentence.

Because 壁紙のつやと差し色 works as one combined object of 見る.

So the structure is:

• [壁紙のつやと差し色] を 見てばかりいる

That means the friend is looking at both of those things as the object of the verb.

Japanese often groups multiple nouns together and then marks the whole group once with を.

Because it is part of the grammar pattern ～てばかりいる.

The sentence is built like this:

• 見る → dictionary form
• 見て → て-form
• 見てばかりいる → keeps looking / does nothing but look

So 見て is not standing alone. It connects the verb to ばかりいる.

～てばかりいる means to keep doing something, to do nothing but something, or to spend all one's time doing something.

In many cases, it also suggests that the speaker feels the action is excessive, unbalanced, or a little annoying.

So this sentence does not just mean the friend is looking. It suggests something like:

• the friend keeps looking at those things
• the friend is focusing on those things too much
• the friend is doing that instead of paying attention to other things

It often sounds at least slightly critical, or at minimum suggests too much of one thing.

So 見てばかりいる can imply:

• the person keeps staring at that stuff
• the person is overly focused on it
• the speaker thinks the behavior is repetitive or excessive

The criticism can be strong or mild depending on tone and context. Sometimes it is just an observation, but very often there is a nuance of always doing that and not much else.

The difference is important.

• 見ている = is looking / is watching
• 見てばかりいる = keeps looking / does nothing but look

So ばかり adds the idea of repetition, excess, or exclusiveness.

Compare:

• 友達は壁紙のつやと差し色を見ている。
  The friend is looking at the wallpaper's sheen and accent colors.

• 友達は壁紙のつやと差し色を見てばかりいる。
  The friend keeps looking at the wallpaper's sheen and accent colors, and that is basically all they are doing.

Yes, and the nuance would change.

For example:

• 壁紙のつやと差し色ばかりを見ている

This means the friend looks only at the wallpaper's sheen and accent colors. Here, ばかり limits the object.

But in your sentence:

• 壁紙のつやと差し色を見てばかりいる

the emphasis is on the repeated action of looking.

So:

• X ばかりを 見ている = looking only at X
• X を 見てばかりいる = keeps looking at X all the time

They are similar, but not identical.

In ～てばかりいる, いる helps create the sense of an ongoing or continuing state/action.

So 見てばかりいる is not just a single look. It means the person is in the habit/state of continually looking.

This is the plain form いる. In polite speech, it would become:

• 見てばかりいます

By itself, 友達 can mean:

• a friend
• my friend
• friends

Japanese often leaves number and possession unstated when they are clear from context.

So the exact English choice depends on the situation. In many contexts, a natural translation would be my friend or the friend if the listener already knows who is meant.

It is in plain/casual style because it ends with いる rather than います.

Plain style does not always mean rude. It is just the non-polite form. It is commonly used in:

• casual conversation
• narration
• written descriptions
• talking to close friends/family
• inner thoughts

A polite version would be:

• 友達は壁紙のつやと差し色を見てばかりいます。