Can you use colorless mana to pay for generic mana costs in a spell's mana cost?

Generic mana costs are represented by numerals in mana costs (e.g., {1}, {2}, {6}) and can be paid with mana of any type, including colorless mana. This is explicitly established in CR 107.4a, which states that generic mana can be paid with any color of mana or with colorless mana.

Colorless mana itself is a distinct category of mana — it is not a color. It is produced by cards like Wastes, Sol Ring, or Eldrazi Temple. Per CR 106.1, the types of mana are white, blue, black, red, green, and colorless. Colorless mana is fully usable for generic costs even though it cannot satisfy a colored pip (like {W} or {U}).

It is important not to confuse generic mana symbols {1}, {2}, etc. with the explicit colorless mana symbol {C}. The {C} symbol (CR 107.4c) requires specifically colorless mana and cannot be paid with colored mana. But generic numerals work the other way — they accept anything, including colorless.

Example: You control a Wastes (which produces {C} colorless mana) and want to cast a Grizzly Bears costing {1}{G}. You tap a Forest for {G} to pay the green pip, then tap your Wastes for colorless mana to satisfy the {1} generic cost. The spell is fully paid for.

So in summary: colorless mana freely pays for generic mana costs. It just cannot pay for colored mana symbols or substitute where a specific color is required.