A liter (spelled "litre" in British English) is the basic metric unit of volume. It's equal to 33.76 fluid ounces or just over one liquid quart. For smaller volumes, the centiliter -- one-hundredth of a liter -- is sometimes used. For very small volumes, such as cosmetics and artists' colors, milliliters are more common.
A standard can of aerosol spray paint as sold in Europe will typically contain 400ml of paint. Acrylic paint, which is often applied liberally to the paper or canvas, is often sold in 59ml plastic bottles. Watercolors are used more sparingly than acrylics and so they are sold in smaller amounts, typically 5ml or 14ml tubes.
"Mil" is one abbreviation for milliliters. The more usual abbreviation is "ml." This is commonly pronounced as "mil," however, and the two are often used interchangeably.
"Mil" is sometimes used to refer to the thickness of a layer of paint or some coating. In the United States, the term "mil" generally means one thousandth (0.001) of an inch, a very small unit used to measure the thickness of a layer of paint or other films such as a plastic coating. This usage must not be confused with milliliters or another metric unit, the millimeter. A millimeter is a unit of length equal to one-thousandth of a meter; there are 39.37 imperial mils to one millimeter.