If I am understanding you right, you're going to take that techno kid fabric and cut it into large squares and want to add colored borders around the square and ultimately join the squares into a top. I would frame the square with 4 colors total -- one color each side.
If you are not cutting up your techno kid fabric but want to add a multi colored border along the lines of the picture you have posted, go for it. The yellow on the top doesn't look as vibrant as the other colors you have and a gray would be a good addition, too. Could just be my computer monitor, too.
Have fun! That is so sweet that you're making a quilt for your cousin's son. You must make great quilts for him to be asking for one. Those are GREAT fabric finds for him. I wish you were my cousin.