Editing the Chart Legend in Numbers

I think I'm not understanding your ask... are you trying to change the color of the Legend? or the color of the data itself (e.g. the bar/line/area on the chart)?

To change the color of the legend text, just click on the legend and use the Inspector -> Format -> Style to change the font/style/size/color to whatever you want:

The colors in the legend reflect the color used on the chart, so to change that you need to select one of the data series on the chart itself, then use Inspector -> Format -> Style where you can choose a different color (and the legend will automatically reflect this):