if you use black in the bobbin and white in the top you (might) get little white dots on the back- and black dots on the top---
if your tension is PERFECT this shouldn't happen- but ...
but that is why people say to use the same- just to avoid that possible problem- it is not always a problem= and many of us use different threads in the top & bottom all the time- it is just easier to have the correct tension when everyting is the same-
your best bet is to make a little sandwich- same fabric-same batting and do some practice quilting- if it works great- if not you can fiddle with it before working on the actual quilt