Can you get brindle boxers from 2 fawn boxers?

This is certainly possible, as long as both dogs have brindle parentage somewhere in there ancestry. For example, if one of the parents was brindle, or maybe even one of the grandparents, they could have brindle puppies. This is because darker colours are more dominant and so are likely to recur again in later generations. For this reason, it is very possible that two fawn Boxers could have a few brindle puppies in the litter. Of course, for a bigger chance of having brindle puppies - use a brindle parent.