please, seamstresses and seamsters- i implore you! don't use contrasting thread to sew your projects together. no mater how you hard you try convince yourself that it's a design element, it's not. For instance- using red thread on a black fabric just doesn't look good.
use a contrasting embroidery floss as an embellishment after you've completed the bulk sewing? Sure. that's a good idea. but doing the main sewing in a color that is not the same as the fabric itself does nothing more than show where you sewed too fast, and where your feed dogs were taxed. Using contrasting thread shows exactly where you strayed from the correct seam allowance, and it shows when your starting point and ending point don't quite match up. it is, of course most obvious where topstitching is required.
no matter what level you are, beginner to intermediate to an advanced clothier- please please please; resist the urge to use thread that isn't the same shade as your finished piece!