Mixing colors from the opposite side of the color wheel (blue and orange, green and red for example) makes brown,so if you mix a bunch of colors together, you are likely going to mix some that are across from each other.

The colour of an object is defined by the wavelengths of light it absorbs. When you add lots of colours together, the absorbtive characteristics of each are combined together and, generally speaking, the darker it gets the more colours you add. If you were to perfectly combine an equal ratio of cyan, magenta, and yellow (incidentally, CMY is the negative/inverse of Red/Green/Blue), you'd get black.

This is in contrast to coloured light, which selectively emits specific wavelengths, rather than selectively absorbing them. In the case of equally combining red, green, and blue light emissions, you end up with white.

In part because 'brown' is a huge range of colours. Everything from dark red to dark green will be called 'brown' if it isn't highly colour saturated. Just about the only colour that won't be is blue, which is a fairly rare pigment in nature

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