Adam Carse's History of Orchestration is available free online. He
will set you straight about the inadequacies of Schumann's and Brahms'
abilities as orchestrators. The quote above is about the "Cathedral
Scene" of the Rhenish Symphony, which has Schumann's best use of
trombones. The first movement is notorious for using the same
orchestral color throughout. I am still waiting for somebody to
record Frederick Stock's rerorchestration.
I think Brahms' orchestral works would be even more effective had the
composer had more interest in instrumental color. Here is what
Rimsky-Korsakov had to say:
"More than one classical and modern composer has lacked the capacity
to orchestrate with imagination and power; the secret of colour has
remained outside the range of his creative faculty. Does it follow
that these composers do not know how to orchestrate? Many among them
have had greater knowledge of the subject than the mere colourist. Was
Brahms ignorant of orchestration? And yet, nowhere in his works do we
find evidence of brilliant tone or picturesque fancy. The truth is
that his thoughts did not turn towards colour; his mind did not exact
it".