To increase the brightness of the rainbow, simply press Ctrl+J (Win) / Command+J (Mac) to duplicate the layer. If, after duplicating the layer, you find the rainbow is now too bright, dial it back a few notches by lowering the opacity of the duplicated layer. You’ll find the Opacity option directly across from the blend mode option at the top of the Layers panel. I’m going to lower mine down to around 70%: