What do you guys call this type of shader or effects? How can achieve this?
I would say it’s a multicolor gradient^^.
Well, one way to achieve this one the fly with shaders is by mixing colors with the GLSL build-in function mix. It essentially performs a linear interpolation between two values (e.g. colors).
Live demo: https://jsfiddle.net/bsp2vqt5/
Of course you can combine multiple calls of mix in order to involve more colors into your effect.