Power ups are in game perks that help you. However some of them don't last long.
BLUE POWER UP Edit
Speedy perk allows you to run faster for a limited amount of time. Your character will have a trail of blue running guy silhouettes behind them while this perk is active.
RED POWER UP Edit
Secondary ammo. Ammo count acquired when picking this perk up varies depending on the secondary weapon equipped.
GREEN POWER UP Edit
Juice/Beer perk gives you health. If you have any item on that gives you more juice/beer power this perk uses that as well. (Xbox uses the word beer / PS3 uses the word juice)
YELLOW POWER UP Edit
Critical chance perk makes every shot a critical hit while you have this perk. Your whole character will have a yellow pulsating glow around him while this perk is active.
ORANGE POWER UP Edit
Melee perk increases your melee strength for a limited time. Your character will have a glowing orange fist while this perk is active.