Earthy tones generally look nice with mustard. If it's 'gold'ish, then light blues are also nice. Deeper shades might look well with oxblood and burgundy.
Mustard is a hard color to match with but i found a couple of choices:lime greenredaubergineemerald greenwhitepeach pinkHope this works out for you!!
Yellow & brown.
Ketchup.
They are colours that do not match
no
white
White and beige are a couple of colors that will match a brown duvet. These two colors are the most common types of colors you will find on a brown duvet.
Image > Adjustments > Match Color
test it
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blue
Yellow and red!