The ColorOperation.Color option makes the drawn object completely ignore its Texture and use only the color. For objects like Sprites, this results in a colored quad (no texture). Alpha is ignored completely as well, so even if the Sprite’s Texture has transparency, it will appear as a solid rectangle when using this color operation.
Sprites that use .Color do not need to have a Texture. Therefore, the following code is acceptable:
Sprite sprite = new Sprite();
sprite.ColorOperation = ColorOperation.Color;
sprite.Red = 1;