That's true.
I believe "Gone with the Wing" is also color.
Or take 1945's "Picture of Dorian Grey", the whole movie is
b&w, except for a special scene towards the end, which is
color.
In fact, there are a couple of very early short silent films,
from France for example, that had some primitve colors
in them.
So the process was known, just expensive as MildPossession
pointed out, and so not worth using except on a very few
big production movies.