It's called a clipping mask.
Hold down Alt and move your mouse over the line that separates the two layers.
When the cursor changes to this , click it.
Note: You can also select a layer and in the main menu do Layer > Create Clipping Mask, or alternatively you can use the shortcut Ctrl+Alt+G
I have a similar issue today.
Fortunately, my elements are all the same color (but separate on different artboards).
My way is:
Edit > Find and Replace Color...
Choose your Find color.
Choose your Replace color.
Tap Replace button.
Video with steps.
GIMP has no spot colour channel support, nor does it support CMYK images used for print separations. To be honest, GIMP is not ideal for print work. Also note that CMYK printing doesn't involve the use of white Ink. So you'd need to check that requirement with your print supplier to find out what's possible. It would have to be created in such a way that a ...
To permanently disable layer boundaries from the File > Preferences menu chose Image Windows > Apearance to untick Show layer boundary. This then works on all newly created layers.
To toggle visibility of layer boundaries create a keyboard shortcut from Edit > Preferences > Interface. Select Configure Keyboard Shortcut and scroll down to the ...