Many paint colours are already mixed and available at arts and crafts stores and paint supply stores. But if you prefer to mix your own colours, you can make various neon colours at home with just flat colours mixed with the brightest yellow paint. When mixing colours, you should be aware that the neon colours are located on the yellow end of a colour wheel spectrum. The following directions are on how to make neon green.
Put a little bit of any blue paint on your brush and dab onto a palette. Choose a blue colour that is in the middle of the blue spectrum on a colour wheel.
Dab bright yellow on top of the blue paint and mix the colours together with the brush.
Add more bright yellow until you get the neon green hue you want. If you accidentally add too much yellow, you can backtrack a bit by adding a little blue paint.
You can make other neon colours the same way by just adding bright yellow. But if bright yellow turns the colour brown, or some other undesired outcome, do not add more yellow. Rather, select the brightest colours that are closest to yellow on the colour wheel. For example, to make neon purple, select a very bright blue and a very bright red. Keep track of approximately how much of each colour you put onto the palette so that you can recreate the colour later, if you so desire.
If you keep mixing your colours but you don't get the colour you desire, it's best to start over from the beginning instead of continuing to add paint.