nuance

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A subtle or slight degree of difference, as in meaning, feeling, or tone; a gradation.
• noun Expression or appreciation of subtle shades of meaning, feeling, or tone.
• transitive verb To treat or consider with nuance; give nuances to.

from The Century Dictionary.

• To shade and give delicate tones of expression to, as a piece of music or a picture.
• noun Any one of the different gradations by which a color passes from its lightest to its darkest shade; a shade of difference or variation in a color.
• noun A delicate degree of difference in anything, as perceived by any of the senses or by the intellect: as, nuances of sound or of expression.
• noun In music:
• noun A shading or coloring of a phrase or passage by variations either of tempo or of force.
• noun A florid vocal passage; fioritura.

from the GNU version of the Collaborative International Dictionary of English.

• noun A shade of difference; a delicate gradation.
• noun A small difference in meaning, significance, or expression.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A minor distinction.
• noun Subtlety or fine detail.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a subtle difference in meaning or opinion or attitude