I have a rather silly question for you all.

If I pour water into a glass of cordial, the colour gets paler as the cordial is diluted. If I pour water onto my pale blue bathmat, it turns to dark blue. Shouldn't the mat turn a lighter blue, too?

From what I know from uni, and what I can find, it's because with solid objects, like T-shirt cotton (or pale blue bathmats!), the fibers absorb water quite rapidly, making the threads much denser. Water absorbs light quite easily (notice that the deeper a water body is, it's darker), so the wet threads absorb more light from the spectrum, causing a darkening. In the case of the cordial, of course both are liquids, so they disperse themselves quite equally among each other. Hope this helps!

Thanks a lot, Reeshy! That makes sense to me...

No problem; although the ellipses at the end of your sentence kind of imply you're unsure? :P