why not use a light blue in the bobbin & your neutral in the top? so it matches? or, you can certainly use a different color (like neutral) in both top & bottom- pull some thread off the spool & lay it across the backing & see if you like it or not- I generally use what ever thread is the right color for the top & then what ever color is right for the backing in the bobbin. just if one side is really dark & one side really light there may be a problem with the little needle hole pokies showing through on one side or the other- but if you are using a fairly light (or the same value) in top & bobbin it usually is just fine. put together a little sandwich with fabric from top, fabric from bottom a piece of batting & practice a little bit- trying out the options you are considering- then you can decide what you like best.