make subject pop?

[echoes]

I have various photos that are watercolor/marker---each photo is mostly a bland white with a color subject. 
I'd like to:

1. convert the bland white background to #ffffff OR transparent
2. make the subject colors more intense

I have several photos, so ideally I do *not* want to manually edit out the subject with the selection tool. 

Any suggestions? 

Here's a sample photo: https://ibb.co/F0RQ9Cd

(Note that the colors here are really bad, adding a sample photo just for illustration.)

[Ofnuts]

Your backrground is a bit uneven (more light on the right). A scanner is better than a camera shot (scanners are cheap), or, next time, overexpose the shot.

This said:

• add a bit of contrast with the Brightness-contrast tool (watch for color changes on the line edges, stop before this)
  
• with the Levels tool blow out the bakcground (drag rightmost handle to the left)
  

   

---

Back to it. Found a way to even out the lighting, and af the same time whiten out the background:

• With the color picker, sample the darker background (bottom left corner) to set the foreground color
  
• With the same tool, sample the lighter backrgound (top right corner). Ctrl-click to set the background color
  
• Add a transparent layer, and set it to Divide mode
  
• Set the Gradient to FG to BG (RGB)
  
• With the Blend tool create a gradient ftrom the bottom left corner to the top right corner
  
• If happy with the result, "freeze" it with Layer>New from visible
  
• If needed you can add some saturation to that layer with Color>Saturation
  

   

[rich2005]

That example, colours are a bit bland to make anything 'pop'
Not the automatic fix I suspect you would like but the Gimp gmic plugin http://www.gmic.eu can get a sort of correction in one go but it is very user interacive, using color curves (CMYK) filter. Looks like this: https://i.imgur.com/5nhF4TY.jpg Had to adjust 3 of the curves
..and the result https://i.imgur.com/wrPfUzf.jpg