Looking at your picture of the quilt, I will say the X quilting MIGHT just be enough if you are talking about only omiting quilting in the very lightest blocks. This is a difficult answer to give without actually seeing the quilt (for me anyway) and without knowing the specific blocks you plan to skip. If it were my top, I would do all of the squares, or at least every other one on each row to maintain a "quilting" balance. (This will distribute the stregnth of your quilting evenly over the entire quilt as well as visually balanced.) Your quilt is done in a multitude of shades of blue, so having the dark blue thread on your light squares will not look out of place. To the contrary, it will most likely blend right in with the scheme of the quilt and not noticed as an individual component of the quilt. This is just my opinion. It is your quilt top so you do it the way you think will look best. Quilting is to hold the 3 layers together. As long as this is accomplished, all the rest is based on your own preferences.
Congrats on finishing the top!