... if you can. It's a weathering thing for the most part. After time, they'll start to blend together. I recommend NOT applying any foreign 'material' to the brick! If you do, you've turned a maintenance-free product into one that will now require maintenance. Then, in due time, you'll be asking us how to REMOVE it!
Brick needs to breath anyway. If you've ever noticed, brick is hard on the outside but soft on the inside. That's because the firing process hardend the brick's outer layer. Also, this final 'state' allows the brick to breath moisture in and out which actually 'gives it life'. If you seal it, well, you're accellerated the aging process too.