All about light, shadows, and contrast.
The process of painting these bulbs may seem simple but it is not. If we add too much water/paint - colors will spread too fast and we may lose the highlights. If we paint it 'wet on dry', the paint will rather dry too fast, and we can easily create a bloom or just hard edges.
So what is the best way to paint these?
It's all about following the steps, and using correct ratios of water and paint.