Spring: your talk about the use of the colour red in this episode made me watch carefully for the colour scheme in this episode. It turns out to be pretty fruitful.
The two characters wearing red are Glory and Anya -- who are absolutely forthright about their intentions, and are both interested, within this episode, in seeking out the truth. (Both have major blind spots overall -- but within this episode, they are the most open to it.) Even Spike, who is telling truths about Buffy's real feelings to Riley, is still not actually telling, or seeking, the full truth -- he deliberately omits details which make Buffy's reasons for spending time with him, and confiding in him, more comprehensible to Riley, and he minimizes the extent to which Buffy does care about Riley (only throwing in "not that she doesn't like you" as a sop).
In case we're tempted to say that truth-telling is all good, Anya is abrasive and often rude, and Glory is so "honest" that she has no concern whatsoever with the comfort or desires of others.
Now, there's blue. Riley wears blue; Ben has his blue medical outfit on under a jacket; Giles has a blue shirt on. Notably, all three characters are associated with blindness to the truth here. To make clear that it's not *just* about Ben being a doctor, Dr. Isaacs is wearing white. You talked about Riley and Giles here; I want to add that Giles' saying "I'm almost certain [Xander is wrong], but to be fair, I wasn't listening" is particularly funny because, of course, Xander was right that Riley is spinning out of control, and it's an issue Giles *should* take some interest in given how directly it affects Buffy. But Giles is both in see-no-evil mode *and* has almost never viewed Xander with respect.
Ben, I think, is especially interesting, though. Ben is Glory, after all -- but Glory, in red, is "the truth," and Ben, in blue, is "the lie." When Dr. Isaacs is asking Buffy questions Buffy is uncomfortable about answering, but which (he claims) may help, Ben interrupts by lying to Isaacs that he's needed elsewhere, and telling Buffy not to worry. Isaacs is fairly callous, but I think I mostly believe that he's a good doctor (it's hard to tell -- that Isaacs asks questions like whether Joyce uses a cell phone makes me pretty skeptical, actually, but I think that's more just an artifact of the difficulty writing convincing medical questions than an in-story incompetence). Ben, who claimed in "No Place Like Home," "I'm a doctor -- well, almost," almost never seems to do any actual medical work, kills patients by summoning a Queller demon, and, despite what I think are at least partially good intentions, basically exists primarily as a way to mask Glory. He comforts Buffy, but his comfort is not just a lie but even interferes with her arming herself and Isaacs arming himself for Joyce's condition. Note: I don't think that Joyce would have survived if only Buffy had answered whether they live near power lines -- but I think the principle, that Ben's "helpfully" interfering with Isaacs collecting information, and that this is bad, remains. Because Ben himself is all a lie -- he is just a friendly spin on Glory -- he makes things worse by making it more difficult for people to see the truth, and the danger (either from death, in Joyce's case, or from Glory, who represents a lot of things, including death).
Why blue? Well, it's not red, so that's one point. It's a cooler, calmer colour, and I think that's probably a factor, too. I think maybe it also has to do with the way they present themselves, kind of, as True Blue.
After he visits Sandy, Riley can't wear blue anymore -- he's no longer True Blue, or able to lie to himself that that is who he is. So he changes into a turtleneck to hide the bite, and, whaddayaknow, it's army green. Next episode he reconnects with the army officially and two more and he leaves.
I think the rest of the characters are more neutral. Willow and Tara are closer to red. Willow has a reddish outfit, though it's more like orange. Not as interested in telling an unvarnished truth as Anya, but in this episode her main activity is to do research, which is an attempt at finding the truth. I am trying to decipher the symbol on her shirt -- it is a heart, with "Elvis" written on top of it, and with a banner that floats around the heart which looks kind of like a snake. Okay, so -- love (red) and temptation? And The King? Dunno. Tara has red underneath a...multicoloured, I guess?...shirt, and is the one who suggests Glory may be older than recorded history and so gets at some truth.
Characters who really don't fit on the scale: Spike is in black, Joyce and Dr. Isaacs mostly in white. Xander has a yellow sweater with a dark shirt underneath, which makes him look a lot like Captain Kirk. Don't know what to make of that. Dawn has a kind of grey and black outfit.
Most important on the red-blue scale: Buffy, and Buffy alone, is wearing a *purple* shirt. Wow. The sweater that Spike picks out is pink (he's attracted to the truth-telling, truth-seeking part of her). But Buffy herself is poised midway between red and blue, between dangerous honesty and kind delusion. She knows she can't accept Riley's, or Ben's, or Giles' attempts to comfort her by telling her everything is okay, but she wishes she could; she knows that she hates Glory, but she also knows she has to seek her out. Really interesting.