Strike the right tone...
1 Lighten the background
Import our start image into Lightroom, then go the Develop module. Set Temperature to 4500, Tint to 0, Exposure to +0.36, Highlights to -14, Blacks to -21, and Vibrance to +26. Next, go to the HSL Panel and click Luminance. Grab the Target tool from the top left, then click-and-drag upwards on the background to increase the Blue and Aqua sliders. Grab the Adjustment Brush from the toolbar.
2 Paint the coloured spots
Scroll up to the Adjustment Brush sliders at the top of the right-hand side panels. Click the Color box and choose a pastel pink colour, then increase Exposure slightly to about 0.69. Go to the Brush Settings and set Size 17, Feather 100. Click in the image to set an edit pin, then click around to add spots of subtle colour, using the square bracket keys to resize your brush tip as you go.
3 Add more colours
Click New at the top, then go to the Color box and choose a different colour, keeping to subtle pastel tones. Click around the image a few more times to add different colour spots, building up the effect gradually. Click New again to set more edit pins for other colours. Instead of the pastel neon look we’ve gone for, you might want to try pastels of one or two colours, such as red and purple.
4 Apply coloured gradients
Grab the Graduated Filter from the toolbar. Set similar colour and exposure settings as before, then drag in from the top-left corner to add a graduated blend that starts off in the corner and gradually falls off. Click in other areas to add more coloured gradient effects, adjusting the colours as you go. It’s best to build up the effects subtly. Take Snapshots as you go, so you can compare different versions.