"I want a wallet in the same color."
I struggled with this somewhat myself, but by now I came to see this use of 「の」 as "further narrowing it down".
「先生」 - a teacher, with no further information
「日本語の先生」 - a specific kind of teacher, being a Japanese teacher
「犬」 - a dog, basically any dog
「私の犬」 - a specific dog, being my dog.
「左」 - left in general
「テーブルの左」 - left of something specific, being left of the table.
The part that narrows it down is in front of the more general part.
In this case, we have
「さいふ」 - a wallet, just any wallet
「おなじいろのさいふ」 - a specific wallet, being a wallet in the same color.
Get me ポテトチップス
Get me 最後のポテトチップス
Be more specific and get me 美味しの最後のポテトチップス
Be even more specific and get me あなたの美味しの最後のポテトチップス
Turn it around and we have: ポテトチップスの最後の美味しのあなた
This could, if we were mega generous, mean: "the last potatochip of the delicious kind which is yours"
So you see, you only need to fool and play around with the orders and you'll ... well, at least get a laugh out of it.
My first thought was the following:
As for the wallet, I want (one of the) same color. My thought was that you were looking at a bunch of wallets and someone asks "which do you want?" Where you specifically want the "same color", instead of wanting a "wallet" (which is obvious since you're at a table of wallets for sale). It seems obvious you want a wallet, so specifying it seems optional, which is where 財布は has the option of being left off comes in. Does my translation work for this scenario, or is it just wrong?
Jisho describes it as a "Noun or verb acting prenominally"
It is an irregular adjective. It can be placed before a noun like an い-adjective but conjugates like a noun
From Imabi: Onaji
Long ago, onaji was like all the other adjectives. It wasn’t the only one that ended in /ji/. All the other ones, though, evolved to end in /jii/ instead. It is for this reason we have adjectives like susamajii 凄まじい (fierce/tremendous) and mutsumajii 睦まじい (harmonious). Had it remained as onashi like it was even further back in time, it might be just like all the other adjectives, but now its conjugations are hybrid between adjectives and adjectival nouns.
From wikipedia Japanese Equivalents of Adjectives:
Attributive onaji (同じ, "the same") is sometimes considered to be a rentaishi, but it is usually analysed as simply an irregular adjectival verb (note that it has an infinitive onajiku). The final form onaji, which occurs with the copula, is usually considered to be a noun, albeit one derived from the adjectival verb.
I think it's because 同じ isn't a na-adjective but a noun.
The exercises in this chapter show that these adjectives; きれい（pretty）、丈夫（sturdy), 色々 (various) can be used in front of a noun if a な is placed in between. Make note that all these are na-adjectives!
Watch out though; there are some exceptions for i-adjectives like 大きい or 小さい which also have な forms!
Some words that in English are verbs i.e. (like, hate, want) in Japanese are in fact adjectives. Or even 同じ being a noun too is slightly strange for me (and even a verb!). Another interesting deal is with the "verb" 要る... I am sure that there are still quite a lot that I still haven't explored. I guess some you just need to remember them.
I'm not 100% on the rule, but I know certain words always use が instead of を, such as 好き, 嫌い, 欲しい, and a couple others. Anytime you want to say you like something, you always use が.
I've also seen を often with things that get acted on, such as を買います (to buy something), を食べます (to eat something), and more complex ones like そこ角を右に曲がります (turn right at that corner).
You use を for transitive verbs, and usually が for intransitive verbs and non-verbs. There are some exceptions such as "角を曲がる", which I believe is because you are turning through the corner. It's really not that complicated actually if you know the rule.
In this case, there is no verb, but 欲しい is an adjective that means "wanted", so you would always use が or は, although は might imply some contrast, along with a few other possible subtle nuances. 買う and 食べる are both always transitive verbs, so you would always use を, and never anything else.
Yes, because you got the order mixed up ;)
「さいふ」 is "a wallet", basically any wallet, with no further information given. 「きいろのさいふ」 for example narrows is down, now we're talking about a wallet with a specific color, being yellow. Of all wallets, only the yellow ones are interesting here.
In the exercise sentence, there's no specific color given, but "the same color" (as something else that's not mentioned). So, we get 「おなじいろのさいふ」.
「さいふのいろ」 would be something like "the wallet of the color", and 「おなじさいふのいろ」 would be something like "the same wallet of the color" ;)
You can always use お before 財布, and the same with any word that you could normally. If the word is acceptable to use an honorific at any time, then it will be for the rest of the time. Honorifics are usually used when speaking politely though, but without context it's impossible to tell (accept for the polite です at the end). Try reporting it so that the contributors can hopefully fix it, and no-one else will have their answer marked incorrect like yours did.