There are 5 Orchids to pick and you will grab all 5 very easily on your way to completing the game and getting to the Underwurlde.
The 5 Orchid colours are:
Blue - Power Orchid, makes you invincible and fast.
Red - Zombie Orchid, makes you invincible and slow.
Purple - Disorientation Orchid, makes you invincible but reverses directional controls.
Yellow - Sickness Orchid, Sits you on your ass, annoying.
White - Cure Orchid, removes other Orchid effects.