Transparent oil paint colors are colors that are easy to thin to an almost see-through consistency. They are not transparent in the way that glass is, but they are less dense compared to other oil colors. They are great for glazing, a technique where you apply multiple layers of thin paint and rub it in. You can mix transparent oil paint colors with white to produce tints, since the clarity of the colors tend to be less muddy than opaque oil colors.
Transparent Earth Colors
Transparent earth colors are used for warm glazing effects. As the name indicates, they often originate from the soil, sometimes from a special locale. Gold ocher is a warm, yellow earth color that imparts a lovely mustard hue. Asphaltum is a color that used to be impermanent but is now created in a new way. It has a charcoal hue and is good for toning down brilliant areas or unifying a region of a painting such as a stormy sky. Transparent mars colors are made synthetically from elements and come in a range of hues.
Phthalo colors, derived from copper phthalocyanine, come in more or less a continuum of hues ranging from sharp green to intense blue. While transparent, they are powerful pigments and a little goes a long way. Different companies make slightly different varieties of the shades. The basic phthalo blue and phthalo green tend to look rather electric and synthetic. They mix well with zinc white to produce very lively tints. They can be used for glazing as well but won't yield the warm or subtle color that is often desired from the glazing technique.
Indian Yellow and Terre Verte
Indian yellow, made from diarylide yellow, and terre verte, made from volcanic sediment, are both extremely transparent. They can be applied in mass directly out of the tube and you can still see through them. They are great color for that reason, as you do not have to worry about overdoing it and covering up an area of your painting that you want to see through. They are almost useless, however, for opaque application. If you want similar opaque hues, you will have to mix them. Start with a sap green for terre verte and yellow ocher for Indian yellow.