css gradient: calculate second stop color

[sdfaheem]

I'd like to be able to be able to calculate the second hex color of a button background gradient based on the first (see sample below). So I would get the first color via a color picker from the user and get that using Javascript (e.g. #ededed in the sample below). Based on the first color, I would like to use JS to calculate an offset which generates the second color and the gradient effect (#dfdfdf in the sample below). The color shift would always be the same, just the input and output hex colors would differ. Is there a formular or function I could use? thanks!\[code\]background:-webkit-gradient( linear, left top, left bottom, color-stop(0.05, #ededed), color-stop(1, #dfdfdf) );\[/code\]