I really need help

This sounds a lot like Image Trace in Illustrator. If you have access to Illustrator, you might try that. You'll find it under Object > Image Trace.

The description sounds like Image Trace in Adobe Illustrator

Dave

This was definitely a Photoshop tool or plug-in?

michaelo52594884  wrote [snip] what I remember it doing was that it had a bot (or something I'm not entirely sure about it) detect all the line segments and shapes on the image and ungrouping them all automatically.

ISTM that it would need to be a vector image, and one created in an app like Illustrator, in order to deconstruct the entire image.  That's unlikely to be doable with an image from a graphic novel, however, even if the GN was digital.  Its imagery is highly likely to be raster with something like JPG compression. 

Having said that, the cartoon nature of a Graphic Novel would lend itself to the standard selection tools in Photoshop, so it is feasible.

The red Avenger in the bottom left corner is an easy selection for instance, either with Select Color, or the Quick Select tool.  From there you can copy to a new layer, and clip a Hue/Saturation layer to the copied layer, and drag the Hue slider to change its colour.  The icon highlighted in green, clips the Hue/Sat layer to the layer beneath it.

Alternatively, if you add a hue sat adjustment layer with that selection in place, the Hue/Sat's layer mask will automatically mask out the rest of the image.

Another approach would be to add the Hue/Sat layer and use its drop-down and select a colour range.  Below I selected the reds for instance, and adjusted the Hue slider.

Note the two colour bars at the bottom of the Properties panel.  The upper bar shoes the original colour, and the lower what each colour range will become.  The white indicators show the colour range being affected, and can be dragged to fine tune.

There are other tools like Replace Colour which is an extension of Select > Colour Range, but I find it way too flaky to be useful.  The selection tools are the best approach IMO.

If you want to show us your project and explain what you'd like to achieve, I am sure someone here will be happy to help if you need it.

Good luck.

It sounds more like you used Adobe Illustrator rather than Photoshop for this activity. Can you check and come back to confirm which application you might have used.