Do you mean the colours on the Budget screen? (I find desktop MUCH easier to work with than mobile, BTW).
Grey just means the category is empty - there's nothing to spend there, but nothing overspent or needed either. It's neutral.
Green means you have money available to spend. It's good.
Red means a category is overspent. This is bad (because it makes it seem like you have more money than you have) and you should fix it ASAP.
The orange/yellowish colour indicates that there's something that needs attention - you might need to fix it, or you might think "Ah right, yep, I know why it's like that - no problem".
That could be:
Overspending (but on a credit card)
You have a goal in that category, and haven't funded sufficiently to meet the goal.
There is an upcoming payment in that category, and there isn't enough money in the category to cover the payment.
For example for me DH gets paid on the 25th but the internet bill comes out on the 29th, so it's currently unfunded and yellow. That's OK because I'll fund it when he gets paid. But I have also just noticed that I have a red bubble in the School Supplies category because DS1 needed €5 for a school book yesterday and took it out in cash but there was only something like 70 cents in the category. I'll go and borrow that now from somewhere else.