First, I would like to point out that this photograph is shot so as so seem mostly blue naturally, likely for power and seriousness.
Angel (David Boreanaz): Dude is wearing black. Spoiler, you probably won’t find a promo picture of him where he isn’t wearing mostly black, which represents “power, elegance, formality, death, evil, and mystery.” Check, sometimes check, sometimes check, check, check this season, check.
Buffy (Sarah Michelle Gellar): Blue pants because seriousness again, white to contrast Angel’s black. White which is “light, goodness, innocence, purity, and virginity.” Kind of check, check, check-but-not to the last three.
Willow (Alyson Hannigan): All you see here is green. Growth and healing are probably the green buzzwords this season, both in relation to her magicks: growth because, oh look I can magick, healing because, hey even though I don’t know it surprise look Angel’s soul!
Xander (Nicholas Brendon): Mostly blue, probably for loyalty and also masculinity.
Cordelia (Charisma Carpenter): More whites and grays, innocence also.
Xander: More blue. Buddy really likes to remind us he’s a guy, I suppose.
Willow: Dark blue, probably for knowledge and power. Again because of magicks, probably, since she is knowledge girl but also she is getting knowledge of magicks, and she is a novice but she is more powerful than she knows. This knowledge/power is contrasted to her almost sheepish facial expression, for example.
Buffy: Appears to be wearing the same outfit as in the above.
Cordelia: Also is wearing the same outfit, though the lighting for this picture reveals its actual color scheme, mostly yellows (dingy ones such as this for “caution, decay, sickness, and jealousy,” which is debatably applicable this season, but there is also an implied normalcy.)
Willow: Green! Also black, but most noticeably green. Definitely back to the growth and healing.
Oz (Seth Green): Black and white, and color theory be damned, it’s just for being snappy.
Giles (Anthony Stewart Head): Brown for stability and masculinity like last week, blue for knowledge. Brown and blue is really the color way of saying “look, I know things that are important!”
Cordelia: Shades of red this time, which on her are visibility, power, desire, strength, determination, and most of the other positive connotations.
Xander: 4/5 of these pictures have had Xander in blue so far. He wears blue inseries, of course, but not this overwhelmingly often.
Buffy: Her blue, however, is daintier, girlier; it almost looks periwinkle. With it and white, this is an “underestimate me at your peril” outfit.
Angel: Black, black, a dark red shirt underneath I think (and a giant, humorous belt buckle, but that’s neither here nor there). Dark red is, in his case, likely malice and wrath.
Spike (James Marsters): Also black. These guys have the same basic color scheme.
Drusilla (Juliet Landau): Black for evil, red for blood, after all they are vampires. Dru’s red also has a more romantic feel to it, one of emotional intensity and extreme passion.
–your fangirl heroine.