One is 14k Gold-filled, one is Argentium Sterling Silver. The wire can also be mixed to use both gold and silver. The outer, twisted wire would be one color, while the inner, smooth wire would be the other.
To find your size, you can measure a bracelet you already own, or you can measure your wrist with a string. Use a tight measurement and then add 1/2" for a comfortable fit. If you prefer a loose fit, you can add up to 1".
Remember that these bracelets don't slide on--they clasp, so they should have a snugger fit.