For each color (white, blue, black, red, and green), check to see if you control a permanent of that color. You can count the same permanent for multiple colors. For example, if you control a green enchantment and a white-black creature, Bloom Tender's ability produces {W}{B}{G}.

Bloom Tender won't produce more than one mana of any particular color. At most, it will produce {W}{U}{B}{R}{G}.

Bloom Tender can't produce colorless mana, even if you control a colorless permanent.