Well, going by the fashion cardinal rule, the color of your belt and shoes should match, of course with little variations (you dont have to kill yourself or others...for that beigish pink belt, coz its next to impossible to get the exact shade in shoes and belt, and yes a little variation for formals and casuals too).
But this rule is far more easier for men to follow. Why? Because women have one more equally significant color in their outfit to work around with - the handbag !
So now there are not two but three equally important elements in a woman's outfit that need to be balanced first and foremost with the garment, and then against each other.
When faced with a situation like this, which is pretty much everyday, I generally follow a few simple tips and of course...go by my gut instinct :)
You can be confronted with innumerable situations, but when its about denim casuals, this is how it typically goes:
Situation A : Classic blue jeans + white top
For me, a white top teamed with blue jeans epitomise a classic casual look, and so I love to team with this look - tan accessories, instead of black, coz tan is extremely chic and equally chilled out. Here, the tan belt and shoes should be more or less the same shade of tan, but the bag can be a little different, say, a chocolate brown with some texture, to break monotony.