Edit: Oil (alkyd) base only. Keep in mind, though they initially look better, the more opaque the stain, the shorter the life.
Bob, is this a typo? I always thought the LESS opaque, the less protection it offered?
Good catch, Mark, but, no. (Kreiss, at least someone is paying attention!)
Color opacity is determined by pigment loading (how many actual grains of the color "stuff" (don't make me get into the "stuff") are in the vehicle (the oil (alkyd) or water base).
The quality of the product is in the vehicle, which contains binders, drying agents, or retarders, etc., and some sort of varnish which will determine the ultimate finish (for paint, not stain). The quality of the color is mostly in the pigment--- quality AND quantity. The more pigment you add, you're taking up space that should be for the vehicle, so the binding properties (above all others) are compromised.
It's the same reason why you can't tint a primer to a deep base color. The ratio of pigment to primer would get all whacky and the long-term performance (mostly adhesion) will suffer.
