litmus

 https://www.google.com/search?q=litmus

Portrait

https://www.google.com/search?q=Portrait

Zilla

https://www.colorzilla.com/ 

My

 https://mycolor.space/

Gradient

 https://en.wikipedia.org/wiki/Gradient

https://uigradients.com/

https://cssgradient.io/

https://www.eggradients.com/