You can't? Your explanation how you *can* is actually very convincing! I could go to my local store, buy 4 or 5 cans of color and do exactly as in your description ... even using Eve as the messenger carrying bottles with mixed colors to and fro.
(Only - what Alice and Bob then would do with their shared secret color, I have no idea ...).

Still, more to the point, an "abstraction" must, IMO, *encompass* the concrete that is derived from it. But modular arithmetic is in no way a *concrete version* of *color mixing*. Therefore, I think the sentence 

*Later, we'll
simply have to describe the concrete implementation of those
functions to get a concrete key exchange scheme.*

should be rephrased e.g. as

*Later, we'll
describe implementations of those
functions that work on data bytes to get a practical key exchange scheme.*

...or... that work on numbers to get ...
...or... that work on numbers to be used as keys to get ...