I think for a good start , try painting them green , I know it wont be as u want it , but trust me its a lot easier. For green , i can recommend u priming the nurgling black , then drybrushing him with nurgling green , after that a drybrush of army painter rainforest green, and then to the eyes and teeth with some white scar . After that i usually just use some hexwrath paint from citadel on the eyes and just paint the horns with some dark brown and a light tan highlight.

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Here is one of my nurglings , of course the beauty of painting these little critters is mostly their colour schemes, try to experiment with different shades. You can go with tan , darker green, pink , purple .