undertone

Short definition from Wiktionary

Noun

1. 1
   An auditory tone of low pitch or volume.
2. 2
   An implicit message perceived subtly alongside, but not detracting noticeably from, the explicit message conveyed in or by a book, film, speech or similar (contrast with overtone); an undercurrent.
3. 3
   A pale colour, or one seen underneath another colour.
4. 4
   A low state of the physical faculties.

Verb

1. 1
   To accompany as an undertone.
2. 2
   To say or speak in an undertone.
3. 3
   To present as less important, noticeable or prominent.