The harder way if you're using Photoshop, but it is more precise:
Select the purple screens. Add a layer style of "Outer Glow". Right click on the layer style and hit Create Layer". Now you have the glows all on a separate layer.
The easier way/If you're not using Photoshop:
Use a soft brush on a low opacity over the linework.
Select the purple screens. Add a layer style of "Outer Glow". Right click on the layer style and hit Create Layer". Now you have the glows all on a separate layer.
The easier way/If you're not using Photoshop:
Use a soft brush on a low opacity over the linework.