Sentence help.

[coco]

A「あなたの　ねこ が くろ でした かね」
B「あなたの　ねこ は くろ でした かね」

Sentence B is used under the special situation.

You can see difference between 「は」and「が」 in those pages.
http://www.guidetojapanese.org/particles.html

We can not say both sentences (A & B ) are past , as netarou-san said.
If a person asks me
「あなたの ねこ は くろ でした かね」
I understand that question means as
"The name of your cat is Kuro, (once I've heard about that, but I m not sure) right? "．
Because くろ/しろ/みけ(三毛)/ちゃとら(茶トラ) are common name for cats.
And commonly people ask the name of pet rather than its color.
We use past tense even if the cat is still alive as same name and same color.
( I don't know if cats can change its color )

Example
「あなたはアメリカ人でしたよね?」means "you are a American, right?"
The past tense is used when we can't recollect though that was heard once.

" Your cat is black (has black color), right? " might be
「あなたの ( かっている) ねこは くろねこ でしたよね」
「くろねこ」= くろ(黒) い ねこ
" Your cat was black , right? " might be
「あなたの かっていた ねこは くろねこ でしたよね」 ( Black cat does not exist at this time)

[InsanityRanch]

ありがとう、ココさん！