I see where you are coming from but personally I would find that too much colour. I like that the red is used to highlight the problem and there aren't too many other colours to distract.
I don't think there is an attribute (?) for a positive value so that we could define it via Stylish such as the negative value. Perhaps
@Alex would consider adding something similar:
{'budget__month-row--negative': month.categoryCache[category.id].balance < 0}
Doesn't NYNAB do all the traffic light colours?