I mix printing ink for a living ( for a food package manufacturing company. We print/make the bags for Frito Lay, Malt-O-Meal, Hostess, etc). While it isn't quite the same as paint, there are some similarities.
One, it doesn't all weigh the same amount when measured by volume. Different pigments are used for different colors, and those weigh varying amounts. Also, different amounts of resins, solvents, etc, are used in different colors. Coverage varies greatly too. A pound of one may "go further" than a pound of another. Mostly due to viscosity and opacity requirements.
For us, white is by far the heaviest color. A 5 gallon pail will hold over 60#, while a 5 gallon pail would overlow with less than 38# of black, or many of our other colors. Our "lightest" color would be our "clear extender", which is ink without any color pigment. 5 gallons of that is only about 34#.
I vaguely remember reading that payload on the shuttle costs its weight in gold to launch. A few hundred pounds of paint = a few hundred pounds of gold?
Paint itself can be costly too. I know some of our 55 gallon barrels of ink cost more than $38,000! That was fancy glow-in-the-dark ink though...
MtnMan