Spider veins are thread-like, as they occur with capillaries that branch off from larger vessels. They are red, blue or purple in color, and they usually appear in clusters and lie flat against the skin. Varicose veins develop in larger blood vessels, so they are large in appearance (usually the size of a garden worm) and protrude on the skin. They are squiggly and blue or green in color. Sometimes they can look like a simple, soft bump in the skin. Varicose veins can cause severe discomfort, heaviness and pain.