How do I remove the background and change the colour of an object (science figure)?

[jrnic1]

Monash University (the institution I work at as a neuroscientist) decided to no longer provide academic researchers (staff and students) with Adobe Creative cloud (essential things like Illustrator and Adobe DC Pro) because they're "too expensive" for the budget. Staff and students are required to purchase said software out of their own wage. Meanwhile, the Vice Chancellor (on AUD 1.3 mil/year) was given a going away party, which cost  AUD 147,000. 

Alas, I need to re-learn how to do all my skills again on free alternatives (i.e., Inkscape and GIMP).

I need to know how to select a colour channel, add a layer mask to remove the background, and then change the colour of a neural spike trace response. here is what it should look like (see images):

[denzjos]

What I tried : 
Remove the white background : Colours / Colour to Alpha
Colour the spike trace : 
- make a new layer
- set a new color as foreground color
- fill the layer with that colour with the 'Bucket Fill Tool' or just click on the 'active foreground colour' icon and move the move to the layer you want to colour
- set the layer mode to 'Screen'
- make a new (white) layer and place it below the original layer

   

Or do you also want to colour the different colours :
   

[rich2005]

If you want to use a mask then this.   https://i.imgur.com/sYRZI4R.mp4

   

[denzjos]

Colours can alse be changed individually using : Colours / Map / Rotate Colours
Here pink changed to blue :
   

[PixLab]

To remove the white background, go to Colors > Color to Alpha...
More about this filter > https://docs.gimp.org/2.10/en/gimp-filte...alpha.html
   

To change color, lock the alpha channel (for image with transparency) with this little button (1), it will respect the semi-transparent as well.
Then just drag and drop any color from the Foreground or background icon (2), don't forget to un-lock the alpha channel (1) or you won't be able to work on the alpha channel, but it can be use to protect it as well
   

To use a mask, go to the Channels dialog/tab, select any channel and drag it below, don't forget to re-select the channel you have selected to drag n drop, because this action will unselect it...
   

Then go back to the Layers dialog and at the bottom there is an icon (mask), click on it a window opens select Channel and Invert if the part you want to keep are in black
   

Result with mask: