Cristina, you don’t have to have a black background, or even a very dark coloured one. Instead of doing a few layers of different sized bokeh, change the Brush variance pallet settings. After you have altered the brightness and contrast, and added a blur, also selected a colour with the dropper tool. For example. Jitter to around 100, colour blend 100, size 2, fade rate 100, impressions per step around 5. Brush setting: size varies, opacity 45 step 18 you’ll get different sizes with the colour. Add a new layer to do a different colour. Once done, blur again if need be.