How can I make omelettes look better?
It sounds like this is more like scrambled eggs instead of an omelet. It does sound tasty, but I can understand your issue with the coloration.

If you're using orange sweet potatoes, that is likely the biggest cause of the coloration issues. Eggs themselves usually stay yellow when cooked, though they may turn brown if cooked and then left to cool and wait around for a while. (I don't normally do that, so I can't really say.) I have a couple possible solutions:

1) As Catija suggests, you can try using white sweet potatoes. They have a similar flavor, though a somewhat different nutritional profile. Still very good for you.

2) Try shredding the potatoes instead of blending them with the eggs. This is probably the solution I would try first. Shredding will avoid mixing the colors to the extent that blending does, and still allows the potato to spread throughout the omelet/scramble. If you do this, I would shred the potato first (peel if you want, though the peel has a lot of nutrients in it), cook the shreds (options: baking, sauteing in a little oil or butter, or microwaving - you can try boiling, but it would be more like parboiling as the shreds would only need a minute or two in the water to cook). Then you can just stir the shreds in with the eggs (no blending) and cook.

3) Slice the potatoes, cook (can cook in the same manner as the shreds, though times will obviously differ), cook the eggs like you would a traditional omelet and layer the egg with the potatoes in your dish.

Here are a couple resources discussing the different type of sweet potatoes available so you can figure out what would work best for your purpose:

http://www.thekitchn.com/whats-the-difference-between-yams-and-sweet-potatoes-word-of-mouth-211176

Paleo/Primal site (I'm not Paleo, but the site has good images of the different types of sweet potatoes): http://www.marksdailyapple.com/difference-yams-sweet-potatoes/