Then I add darker washes to further accent details, followed by dry brushing with a lighter color for highlights. I've noticed that hulls and splinter shields of ships that have been at sea for extended periods are lighter on the bottoms. I've finally concluded that this is from buildup of salt deposits- spray hits the plates and the water evaporates as it drips down leaving salts and mineral deposits on the lower portions. The same goes for hulls where the swell runs along the waterline. Obviously less pronounced the higher you get.
Decks have traffic areas where the paint is kept closer to fresh by the wear from the crew's shoes.
Non-traffic areas have more oxidation and are lighter. Runoff from the upper decks will stain the lower decks, too. I like to use water colors. I have a 10-color kid's set that I use all the time. Just mix black, browns, white, yellow and orange in varying amounts using the inside of the paint set lid for a palette.