One necklace, two ways:
 
Right: A dove grey ruched dress forms the basis of this simple outfit, accessorised with strappy sandals and a beaded clutch in soft shell pink.
 
Left: This outfit comprises a softly-pleated mid grey dress, classic round-toe pumps in ballet pink and a heavily embroidered clutch in gold tones.
 
None of the colours used recommend that green be added, but this green necklace works because the hue is of the same tone as the other colours, so nothing is overpowered.

Brock - Pewter Gym leader, campfire cook extraordinaire, and would-be ladies' man. Brock is a down-to-earth type of person, so this is a practical look, perfect for a full day of classes or fieldwork.
 
The basics: This is designed for a warm climate. Opt for natural fibres like cotton, as they breathe better than synthetics, and pack a parka in case of inclement weather. This also give you stacks more pockets - practicality! Sand-shoes are casual-cool and comfortable if you're walking from one side of campus to the other.
 
Colours: I've chosen a colour palette of mostly neutrals and greys, as Pewter City is known as 'The Stone Grey City'. By keeping to only a few shades it's easier to look pulled-together and fashionable. Brock himself likes to mix it up a bit, though, so don't be afraid of adding some colour, like this pumpkin t-shirt. If you're also a failed-romance waiting to happen, or you just want to express your fandom, try something like this bleeding-heart t-shirt, in breathable and soft viscose.
 
Accesories: Brock isn't really a fancy jewellery type of person, so I've kept accessories minimal - basically sunnies and a rucksack. If you do go the jewellery route, maybe take inspiration from the Boulder Badge: simple, geometric, silver or grey, and stone. The pendant necklace is a subtle reference. Alternatively - see those earrings? They're little bats! They have smiles and giant teeth! They're so cute! They also look a little bit like Brock's Zubat, which is cool, and referential, without making your teachers think you're not serious about your studies.
 
Let me know what you think!