I knit it a week and a half ago, out of some crochet thread that I had just finished using in another HP-related project. It's basic size 10 crochet thread I found at thrift stores; I don't remember why in the world I bought the red, but when I saw the yellow-gold I snatched it up, because aside from its being a great color in its own right, I knew it would work with the red as Gryffindor. I see crochet thread in thrift stores fairly often, and it is a good bargain because it holds up well (IME thus far, old crochet thread in thrift stores always looks better than the old yarn) and because it lasts for a long time. From my ball of red and gold--29 cents and 25 cents respectively--I've gotten one relatively densely knitted project, one headband, and a passel of crocheted bookmarks. There's still a fair amount of thread on the gold ball (which said 175 yards on the band) and a lot of thread on the larger, red ball.
Anyway, if anyone wants the very simple pattern, here it is.
House Colors Headband
size 10 crochet cotton in house colors (red & gold Gryffindor; blue and bronze or silver Ravenclaw; yellow and black Hufflepuff; green and silver Slytherin)
US size 3 ( 3.125-3.25mm) knitting needles
small crochet hook
tapestry needle for weaving in ends
N. B.: Headband is worked holding two strands of thread together.
Start of headband:
Using crochet hook, ch 50 in MC, then switch to knitting needle.
CO 1 (now 2 st on needle) and K across.
CO 1 (3 st on needle) and K across.
K 1. Kf&b. K across.
Repeat last row until 18 st are on the needle, or until piece is desired width.
K three more rows.
(K 3, P until last 3 st, K 3; then on next row
K across) Repeat this until you have 14 rows of st st.
Join in CC, work 2 rows of st st, continuing to have the 3 knit stitches on each edge throughout.
Join in MC, work 4 rows of st st.
Join in CC, work 2 rows of st st.
Join in MC.
Repeat this pattern until you have 4 repeats of the trapped bar design.
Work 14 more rows st st in MC.
End of headband:
K1, K2tog, K across.
Repeat until there's only one st left on needle.
Switch to crochet hook and ch 50. Tie off.
Weave in ends. Iron or press headband to block, if desired; you may also spray lightly with spray starch.
Notes for alteration: Size 10 crochet thread, when doubled, is approximately equivalent to size 3 crochet thread (just a bit smaller); consequently, if you can find appropriate colors in size 3 thread, it will substitute almost exactly. This headband is pretty wide, about two and a half inches, so some people might prefer a more narrow scarf. If making this for children, you'd probably only want three repetitions of the trapped bar design; it would be easy enough to make the crochet chain ties a little longer if three repetions is almost long enough.