"באיזה צבע הבננה?"

Translation: What color is the banana?

[Leshonim]

Why do you need to use a preposition before איזה here?

[NaftaliFri1]

It's like saying "in what color"

[Hannah649004]

Could you just use "איזה"?

[tuki12345]

you can also say
מה צבע הבננה?
what is the color of the banana
but if you want which - that's איזה

[alexkrule]

Is it common to have the ב before איזה in this case?

[giladxw]

Absolutely. It would sound weird (or slang) without the ב.

[NaftaliFri1]

More slang than weird IMO

[tuki12345]

I'm not sure when it's correct and when not, but sometimes ב is not suitable and sometimes required (or slang/not good hebrew without)
איזה יום מתאים לך?
which day is good for you?
באיזה יום נלך?
on which day should we go
איזה צבע אתה אוהב?
which color do you like?
באיזה צבע להשתמש?
which color should I use?

idk which is used when ><

[NaftaliFri1]

According to your examples, it seems איזה is used to find out something, while באיזה is used to decide.

[tuki12345]

HHmm yes I think yometimes but
איזה צבע אתה אוהב
And
באיזה צבע להשתמש
Should be identicle in that sense and they are not in the use of ב

[NaftaliFri1]

One is asking of a color already chosen, the other asking to decide on a color.

[tuki12345]

Still
איזה צבע אתה אוהב ירוק או כחול
באיזה צבא תבחר ירוק או כחול

[TeribleT]

But they used a bet in the example, and that was to find out.

[D.EstherNJ]

Green, then yellow-green, then yellow, then yellow with black spots, then finally black.

[genistv]

That's another of the occasions where I discover I don't get English prepositions. In whaf color is ok but of what color isn't?

[Ringcycle]

In English, we almost never say "In what color" or "of what color". We usually just say "What color is it". I think the comments above were trying to explain why the bet was used, showing that it means in or of, even though they aren't present in actual spoken English most of the time.

[NoeRub1]

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