How can I edit an .SVG file?
An .SVG file IS a source file. It isn't layers in the Photoshop/Gimp sense but it absolutely can be picked apart. Use an SVG editor - that would be Illustrator or Inkscape.
Alternatively, if you want to get real crazy you can open the .SVG in any text editor and look for the values you want to change which for colors would be in Hex format #nnnnnn