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.
white
They are colours that do not match
no
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
Light or paint? I know for sure that yellow light is made of red and green light...so to get mustard yellow, just use different hughes of those colors. As for paint, and I'm guessing here...yellow and some brown.
Blue
Yellow and red!