[chris (OP)]

Water flicked onto a wall, paint that is not dry and freshly-washed hair are all darker than their dry equivalents. Why?

[guest39538]

Water flicked onto a wall, paint that is not dry and freshly-washed hair are all darker than their dry equivalents. Why?

Waters more reflective than the wall so the wall becomes a blacker body that can be seen through the transparency of the water?

In simple terms the water creates a barrier?

[RD]

... freshly-washed hair ... darker than ... dry ...

Water-coated-fibres will be similar to fibre-optics ...

http://www.thenakedscientists.com/HTML/experiments/exp/water-fibre-optics/

The internal-refection will repeatedly redirect some of the light back towards the fibres where some will be lost as heat. These additional reflections from fibres, (losing some light-energy each time), would not have occurred if the fibres were dry.