When undertaking a necklace design, people have asked many times, 'how do I know what goes together'?  There are three rules to follow:  1) Choose COLORS that compliment, contrast, or make the design 'snap.'  Consult a color wheel if you're stuck.  Generally, three colors work well, perhaps four, if you count the metal tone of the findings as one of the colors.  2) Pay particular attention to the PATTERN of the beads and try to pick up one of the patterns in the findings or center piece of the necklace.  Try to avoid too many patterns at once, as they will draw the eye away from the design, as opposed to drawing it toward it.  3) Watch the TEXTURE.  Beads are tactile.  Humans have loved to touch them for 10s of thousands of years.  While there may be some who prefer to keep all the beads "shiney," or "matte," experiment with variation.  It can work!  And, be sure to add some beads that simply feel different than others in the design.  Good luck!