GIMP :: Way To Repeat Exact Same Colors On Separate Image

I am working on tiny comic-strip frames that are being drawn as seperate images and when it comes to colouring, there is a time-consuming snag where I forget the exact colours of the finished frames which I need to keep checking and seeing earlier single images in order to compare and then reproduce the colours accurately in a consistent sequence.

Believe it or not, I get around this problem by taking digital still camera shots direct off the screen and consulting them afterwards! This does work, but it's hardly practical------is there an easier way to repeat the exact same colors on a separate image, without having to constantly refer to pre-existing images within GIMP? If I can reproduce the exact colours on different images automatically without having to recheck earlier images, this would save me a lot of time!

[Ideally, does every shade of colour have a code number or similar method I can enter and it will come up again accurately? This way, I could write down the shade number and repeat it exactly later on a new image where the exact same colour is required.]

The sheer number of shades of colour and tones make it very difficult to remember the exact colours accurately.