term.color
image/color style model for 256 color terminals

To convert color.Color to closest 256 color terminal value you can do:
// var c color.Color
val := color.Term256Model.Convert(c).(color.Term256).Val
or you can get closest greyscale value (intensity) with
// var c color.Color
val := color.Term256GreyscaleModel.Convert(c).(color.Term256).Val