Color Codes

I am having difficulty figuring out what the correct color codes are, especially ColorFieldInterpolator. It seemed like I could just use hex color codes but that seemed to be completely off. I wanted this color (a gray). 

However, when I use it as a keyValue in the ColorFieldInterpolator, I am given shades of blue.

Is there a color dictionary or a way to convert colors to this 'format'? What format are these color codes?

Thanks