Iridescent Shader - Blender (Patreon)

Someone asked to see my iridescent shader for the shiny birb model. In this case it's just a quick faked effect, it doesn't calculate light paths or anything, it just gives you the colourful colours if you're looking directly at a surface and fades away to the proper body colours as you get more off-angle.

Take whatever is driving your main body colours and mix it together with a colourful colour ramp- play around with whatever colours you want and where they sit on the ramp until you like the effect. Mix the two together using a "Layer weight > facing" node, play around with the value until happy, and if you want you can use an additional colour ramp in between the layer weight and the mix node to control how strongly the colour is mixed in. You can take the black handle of the colour ramp and change it's colour to be whiter to get a more subtle mixing effect.

Then output that to your BSDF and bam, you're done!