What colour riding breeches should a man wear?

Whatever colour they like for schooling/hacking/pleasure rides/yard work.

For hunting beige unless wearing top boots and scarlett hunt coat when white is correct.

For competition check the rules for your discipline, but generally beige is safe for everything.
 
Any you want. Dark colours are more forgiving for yard work and exercising.
For competition it would depend on the association or discipline. White is most common, though I believe any plain neutral colour is allowed.
 
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