Here at the store, we generally steer people from this concept. The quick answer is that while one coat may 'look' like it covers, that second coat is where the magic happens. The second coat gives you a thicker film build, which means you have more of that color for the light to reflect from.
But I wanted to dig a little deeper. So I contacted Tom Hill, president of C2 Paint and general paint chemist wonk, and asked him for some more clarification. Here is his input:
One Coat Hiding in Paint
To try and simplify this, hiding in paint is determined primarily by refractive index and absorption properties of the pigments used in the paint formulation.
Light passing through a media is refracted (bent) depending on the
refractive index of the specific media. The refractive index of air is
1.0008. Light is essentially not bent in air. The refractive index
of titanium dioxide is 2.7. The only material with a higher refractive
index than titanium dioxide is diamond. A pigment with a higher refractive
index hides better than a pigment with a lower refractive index. The
refractive index of bright yellow colorant is 1.4 and thus a color made from
primarily bright yellow colorant and zero titanium dioxide does not hide very
We use Titanium Dioxide as the prime hiding pigment in paints because
it has the highest refractive index of the pigments available for use to produce
a white color. However, there is a practical limit to the amount of TiO2
one can use in a gallon of paint. Increasing the amount of TiO2 beyond
this practical limit actually reduces the total hiding of the paint.
At this practical limit…with reasonable volume solids, PVC and dry film
thickness…we expect hiding to be at a minimum of 98% but less than 100%.
The definition of one coat hiding in the paint industry is 98% coverage over a
black and white contrast ratio chart. To increase the hiding of a white
base from 98% to 100% we must add a color pigment that absorbs light as well as
refracts light. The pigments that do this are black, yellow oxide, red
oxide, burnt umber, etc. Once there is a sufficient loading of these
pigments plus white true one coat hiding can be achieved. The colors are
all muted and earth tones but they will cover in one coat.
An interesting side note is that adding bright yellow or bright red to
a white base will reduce the hiding of that base. These pigments do not
absorb light and their refractive index in significantly less than TiO2 (1.4
versus 2.7). The blend of these two pigments yields a refractive index
that is less than that of the untinted base and thus poorer in hiding.
Conclusion, in normally formulated paints (i.e. proper PVC…volume
solids…and film build), one coat hiding is not possible to achieve unless the
paint is tinted with a light adsorbing pigment.
I hope that clears it up!