I buy the great majority of my yarn online, and to try to get an accurate idea of the color, I will often find the same yarn on several websites and compare the colors in the photos. There can be a lot of variation! WEBS and purlsoho.com usually have large and luscious photos of yarns (not always a guarantee of color accuracy, however). I should probably buy a selection of color cards; at this time the only ones I have are for Louet yarns.
Anyhoo, I had the opportunity a few weeks ago to visit one of my go-to LYSs, Elissa's Creative Warehouse in Needham, MA. I had my orders: one son wanted red, the other, orange. I was thinking something like Brown Sheep Cotton Fleece: a wool blend, relatively durable and easy to care for-- since we're talking about a designing a vest for a 5- and an 11-year-old boy. The Cotton Fleece was there, along with many, many other choices, but this is what I ended up with: Adrienne Vittadini "Trina", a hand- or machine-washable heavy-worsted blend of merino, microfiber and cashmere. Certainly not the cheapest, nor the most durable yarn available; I chose this simply because I loved the colors, both the red and the orange (the colors in the photo above are pretty close to the real thing). The red is a vibrant true red, shading neither into pink nor purple, and the orange is a deep, crisp orange, without a hint of sherbet or neon. One of the great advantages of buying yarn face-to-face! (Oh yes, and it's also very soft.)