MakeColorTransparent()

void MakeColorTransparent(Color color, float amount)

Scans all pixels and matches them with color. The more the pixel and the color are alike, the more transparent it will become. amount is a value between 0 (no transparency) and 1 (full transparency).

void MakeColorTransparent(Color color, float amount, float tolerance)

Same function with more control over the amount of transparency and the tolerance used when the color doesn't exactly match.
amount is a value between 0 (no transparency) and 1 (full transparency).
tolerance is a value between 0 (small tolerance) and 1 (big tolerance).