“Changing Colors,” a rewardingly eclectic concert delivered by the Chamber Orchestra of Boston (COB) under David Feltner at First Church in Boston on Saturday, was apt not only for the imminent autumnal equinox but also for the featured repertoire ranging from Mozart to a world premiere with composer present. The ensemble consisted of nine players: Hsin-Lin Tsai, Colin Davis, Heidi Braun-Hill, and Heather Braun, violins; Joan Ellersick and Dimitar Petkov, violas; Rafael Popper-Keizer and Holgen Gjoni, cellos; and Irving Steinberg, contrabass.

We began with Gustav Holst’s Brook Green Suite. One wouldn’t guess that this lovely music was written in 1933, quite near the composer’s death; its graceful tunes and conservative harmonies place it firmly in the English pastoral mode. The fresh-faced Prelude was given a suitably chaste reading though the deliberately minimal vibrato did expose momentary minor lapses of intonation now and again. Air was an enchanting sicilienne with a wistful tune, a direct descendant of Greensleeves. The concluding Dance was an energetic jig which was never less than enjoyable but might have benefited from a bit more abandon.

Progressing from green to blue, as it were, Feltner and the COB gave the first performance of Azure for String Octet by Adrienne Elisha. Described by the composer as “a compelling world of pure sonority which is constantly changing and evolving,” it built from a quiet bare-octave opening to a central polytonal climax in the upper reaches of all the players’ ranges; the sudden silence that followed it was powerfully cathartic, in the manner of Samuel Barber’s famous Adagio. Suggestions of melody appeared from time to time as did striking string glissandi of varying size and speed. Ultimately, the work tapered to a hushed conclusion “with the same feeling as in the beginning: that of the mysterious and magical—and a music which has always been playing before and continues—into infinity.” The composition and performance demonstrated the wide range of playing techniques, both conventional and otherwise, available even to an ensemble of purely string instruments. Elisha and the audience showed the performers their warm appreciation.

Wolfgang Amadeus Mozart wrote his Divertimento, K. 173, when he was 16 and still in the service of Archbishop Colloredo of Salzburg, but already the young composer was including touches that must have raised some eyebrows, e.g., the very opening sequence of descending octaves to a diminished seventh chord. Feltner led the COB in a polished and expressive Andante as well as a rousing, crisply played Allegro di molto. The concluding Allegro assai was ever congenial if a tad restrained for a piece called “divertimento.” I was grateful that Mozart’s indicated repeats were observed throughout.

The resplendent colors of Ottorino Respighi’s Il Tramonto made it a natural choice for this program. This setting of Percy Bysshe Shelley’s The Sunset—translated to Italian—for medium voice and string quartet on this occasion was played by a double quartet which, excluding a few moments of imperfect tuning, worked well. Mezzo soprano Krista River gave a profoundly touching performance, moving seamlessly between recitative and arioso while subtly delineating narration and characterization. In the tale of a young couple whose love is cut off by the sudden death of the boy, the mezzo convincingly portrayed ecstasy, serene bliss, shock, grief, resignation, and (just possibly) hope. The players and River made deeply poignant the physical description of the girl’s pining away, the strings’ texture and sound thinning out, harmonies simplifying, and the singer maintaining an exquisite, otherworldly piano fairly high in her range. Perhaps the peak came near the end with the girl’s quiet entreaty to share the boy’s one-word epitaph—“peace”: River delivered a heart-melting messa di voce on the crucial word. This performance will surely resound in many memories for a long time.

The evening concluded with the Octet, Op. 17, of Niels Gade (1817-1890), the foremost Danish composer of his day. If Gade preferred a more Continental style than did his fellow Scandinavians, Grieg and Sibelius, it was no doubt due in part to his residence in Leipzig in the mid-1840s, serving as Mendelssohn’s assistant conductor at the Gewandhaus Orchestra. (It was Gade who conducted the world premiere of Mendelssohn’s famous violin concerto.) The COB players warmly embraced the handsome, flowing melodies of the opening Allegro as well as its harmonic surprises. The Andantino encompassed a melancholy first section exploiting the lush textures offered by eight voices, a jaunty passage with dotted rhythms, and a charming (one stray pitch excluded) pizzicato ending. Possibly the only true display piece on the program, the scherzo movement was unified by an anapest rhythmic motif and featured a whizzing triplet accompaniment figure. The final Allegro vivace  was less about technical brilliance than a well-rounded conclusion to the large-scale work. To be sure, vigorous high spirits predominated but did not exclude contrasting moments of delicacy. For a performance full of “changing colors” featuring a vocal gem masterfully sung, an under-served composer, and a living one, the COB and David Feltner are to be applauded.