Complex sentence: As the novel opens, the Derwatt enterprise is threatened by a disgruntled American collector, Thomas Murchison who (correctly) surmises that one of his paintings is a forgery due to an untenable reversion in technique involving the use of the color purple.
Split into simpler sentences: As the novel opens, the Derwatt enterprise is threatened by a disgruntled American collector, Thomas Murchison. Murchison (correctly) believes that one of his paintings is a forgery due to a change in the use of the color purple.