The current AKC Papillon standard just might be one of the more precise breed standards when it comes to color and markings. It indicates not only that the breed is always parti-colored or white with patches of any color(s), it goes to specify where those colors and patches may appear. To paraphrase: Colors other than white must cover both ears on the back and the front. That color must extend (without interruption) from the ears over both eyes. While a clearly defined white blaze and noseband, along with symmetry of facial markings is preferred, a solidly marked head isn’t disqualified, but, an all white dog, or a Papillon with no white at all is (disqualified).
At one time, however, “dwarf spaniels,” (an early name by which Papillons were known) were often solid-colored. These days, no two Papillons are exactly alike because the possibilities for color and markings are nearly unlimited; there are five acceptable markings and five acceptable colors for the breed (and ten more colors not acceptable in a conformation ring). One of them is the liver sable which to an untrained eye looks like a standard red or red sable Papillon. This is a dilute color and genetics of this color also impact nose and eye pigmentation of the nose and eyes.
We came across a conversation re: solid colors in a breed forum, and as there is no copyright mark on the page, we infer that it’s safe to share the passage of interest: