Changing the colour of Multiple LED lights on a Christmas Tree

One way that you could make the lights white and keep the tree green would be to add the Black & White adjustment layer and use Color Range to make the adjustment layer's layer mask show only the green of the tree. Here's how:

1. Add the Black & White adjustment layer by clicking on the center icon at the bottom of the Layers panel and select Black & White... or go to Layer > New Adjustment Layer > Black & White... The adjustment layer will make the image appear black & white. Adjust the sliders if you'd like to make the colors that correspond to the light colors lighter or darker.
2. Select the layer mask of the adjustment layer
3. Go to Select > Color Range...
4. Use the eyedropper to select the green of the tree branches (hold down Shift to add to the selection)
5. Select Invert and click OK