If you add more of that striped fabric, it won't look so out of place. Since you are placing them one by one, you can be careful that you don't get two that are too close in value next to each other.Take a black and white photo of your layout, and you'll be able to see which are too close in value, and end up looking like on giant diamond. You are definitetly on the right track. Add some unexpected colors (yellow, pink?) or darks for interest. Looking forward to seeing the finished quilt!