Wikipedia has an extensive set of anatomy SVGs, but I can't highlight different organs or parts dynamically, because there are no source files with layers I can manipulate in Photoshop or GIMP.
How can I edit an .SVG file?
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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
The TL:DR; solution to this is to use Inkscape: https://inkscape.org/
It's free, open source, cross platform. You can install on Ubuntu with "snap install inkscape" or use the installer on the website for PC and Mac.
If you have the Adobe suite (and there's a 30 day free trial) you can use either Photoshop paths or Illustrator to import, edit and export SVGs. When I use Photoshop, I switch my Quick Export preferences so I can right click on a shape layer and "Quick Export as SVG".
SVG vector graphics may also be hand edited in any text editor, as they are essentially just a markup container for a object containing a series of vectors.
One other thing to note: SVGs use markup language, specifically the and tag, and these tags are accessible for styling via CSS. For example, you can give a path an ID and assign it a fill color.