Can I make my stamped concrete a uniform color if there are pieces that are different colors?
There are 3 options:

1) Solid Color Concrete Stain: Just like solid body stain for wood, this is provides a uniform color and should cover the two different color stains you have. However, many feel it looks too uniform. In fact, most complain that it looks "plastic". Stamped concrete should look like natural stone rather than painted stone.  (A good solid color concrete stain is made by Behr.) 

2) Extreme Concrete Restore: This is a restoration Stain and is similar to Solid Color Concrete Stain, but works well for older concrete surfaces too. (Rustoleum makes a good product.)

3) Semi-Transparent Concrete Stain: This creates a more natural stone appearance. However, when it is applied over two different colored concrete stains (like yours,) the different colors will show. Therefore, you'll need to first match the two colors up first by staining (solid stain) one of the colors...probably the darker color. 

Semi-Transparent Concrete Stain comes with two different colors of Stain. One is the base color and the other is the accent color. Natural stone is not uniform. Therefore you'll need to apply the accent Stain over the base color with a brush to make it look natural and not so "plastic". (It takes some practice.  But don't worry, if you don't get it right the first time, you can go over it and do it again...but the color will be slightly darker.) If the base color is a little light, you can apply a second coat to darken it. 

If some of the Stain is newer or some is sealed, you may need to give it all an acid wash. That way the concrete will "take" the Semi-Transparent Stain more uniformly. (To check to see if the various sections of concrete are sealed, pour about 1/4 cup water on it and see if it puddles or soaks in. If it does not soak in within 2-3 minutes, then it is sealed.) There is no reason to remove the sealer, except to have the concrete all the same. If it's all sealed, I'd skip the acid wash. 

Remember, making the concrete look like natural stone is more artistry than a procedure. It takes patience and practice. If it doesn't turn out and you're too frustrated to try it again, you can always use the Solid Color Concrete Stain. Option 3 is the most difficult, but gives the best appearance, when done correctly.