a) for jeans, try a brown belt like the picture below, or you can go with your typical formal belt which you use for your dress pants

b) i would say for that specific shirt which he wore (by cityofpalms), its better to go without a tie, it's clear he's unsure of the look he wants to go for, on one hand he wants to look super chic and youthful with that printed shirt and funky belt, and on the other hand he wants to look like a gentleman, with the tie and the jacket and the bag.. it just doesnt mix right..

c) for that kind of shirt, thats like a really really casual shirt as there are big prints on them, but actually what determines whether it should be tucked in or out is not about the design of the shirt, but the cutting.. the regular dress shirts you see in the department stores which guys use for work - those are meant to be tucked in, lots of guys actually wear those shirts untucked - very ugly look.. shirts which are meant to be untucked are cut short (because they are designed to be untucked), see example below:

a) Belts do not suit the trousers, they suit the shoes. Belts can be of any colour as long as they match or well coordinated with the shoes. My favourites are black, brown, white and other dark colours. Bright / neon colours are out of the question for me.

b) If you are talking about cityofpalms' look, no tie can ever go with that printed shirt.

c) Depends on the shirt's length and fit at the waist. Shirts with shorter hems or fitted waist can be worn untucked. The opposite goes for normal length or loose fit waist.