Hope I'm not too late responding.
Not sure if you found a solution for the background color for the quotes, but look in your admin panel, under
Appearance, in
BBCode Elements, find
BB code blocks. Under
Background, is where you set the color.
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I picked
Alternate content background color. It's part of Xenforo color convention, not an hexadecimal in this case.
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I don't know how you could change the font color specifically for the BBCode quote, though.
Good luck!