code: func (dc *Context) SetRGBA(r, g, b, a float64) {
  	dc.color = color.NRGBA{
  		uint8(r * 255),
  		uint8(g * 255),
  		uint8(b * 255),
  		uint8(a * 255),
  	}
  	dc.setFillAndStrokeColor(dc.color)
  }
docs: // SetRGBA sets the current color. r, g, b, a values should be between 0 and 1,
  // inclusive.