I use Decobob in the bobbin on my quilts. It is an 80 weight, so it is finer. On baby quilts I use regular thread.
You can use different colours front and back, but you have to be careful with that because your tension has to be pretty close to perfect...the threads meeting in between the layers, otherwise you'll get the bottom thread showing on the top or vice a versa. Not a real problem if you have a wild pattern on the back and have a bit showing, but a real problem if you have a white top and end up with your backing thread being dark and showing up on the front.
Watson