Jakko makes several excellent points about the use of modeling specific paint and paints made for other purposes. Sure, the price per gallon of modeling paints seems outrageous. In my retired professional opinion, it is at least faintly outrageous—but there is considerable justification for the price. The first, and from the builder's standpoint the most important, is color accuracy.

Color matching paint is a mixture of art and science. It used to be all art, but computers and technology have made it more science and less art, nevertheless the art is still there. Color matching, called shading in the paint industry, using only the human eye, is an art. When did you last have your color vision checked? It varies from person to person, and changes with age. If you have a cold or flu, it also changes temporarily. The human eye is a wondrous thing, but human perception is easily fooled.

The paint technician has access to a very wide selection of very precisely made pigments. Small amount of these have a significant effect on the shade of a color. To get the same effect by mixing paints is very inefficient and likely to be quite wasteful. Can it be done? Sure, but to you want to build and paint a model or shade paint? It's one thing to add a bit of white to lighten a gray to look faded, quite another to match a Federal Standard color.

The second (of many) is pigment size. Model paint pigments have a very small particle size, enabling good coverage with a thin coat that doesn't obscure detail. Paint made for other purposes will have much coarser pigment, resulting in much thicker coats for the same coverage.

A lot more energy and time goes into the production and packaging of a model paint than a can of Rustoleum™. Part of what you are paying for is less of your time messing about with paint.