code: func (c *Canvas) SetColorMask(col color.Color) {
  	rgba := pixel.Alpha(1)
  	if col != nil {
  		rgba = pixel.ToRGBA(col)
  	}
  	c.col = mgl32.Vec4{
  		float32(rgba.R),
  		float32(rgba.G),
  		float32(rgba.B),
  		float32(rgba.A),
  	}
  }
docs: // SetColorMask sets a color that every color in triangles or a picture will be multiplied by.