It can be really hard not to lie, and adults do it all the time. I have, as recently as a couple of days ago, lied to keep my mother from feeling bad. I'm sure I could think of more examples of my own if I spend only a few minutes pondering.
Before you decide how much and what kind of energy to put into punishing your girls, I hope you will read this most enlightening article on why kids lie (http://nymag.com/news/features/43893/). It's not great to lie, of course, but if you look at the example you have from your children's point of view, it may be that they were unexpectedly cornered into admitting a 'crime' that was, at the time it was committed, not a crime at all. In their minds, they may have been having fun during a boring day decorating the inside (not even the outside) of bins that belong to them.
Why, in a child's mind, would that necessarily be wrong? Where's the harm? It doesn't meet your aesthetic standards, but would they have a way to know that ahead of time? But then, big surprise, they found out it pisses you off. They didn't know you'd be pissed, and they want you desperately not to be pissed, and they don't want to be in trouble. And of course they did it, so why would you even have to ask? The very first thing that pops out under that stress is "I don't know," while their little brains are helplessly sorting through everything else they might say. It's only when you become additiionally enraged at their lame distancing that they realize there's nothing left to do but 'fess up.
I can recall being in that position a hundred times or more with my own mother. She had expectations that I discovered the hard way, by violating them. They were unspoken until then. Each time, I remember feeling a huge collapse of any hope that I could please my mother or avoid the inevitable, shaming lecture/spanking/lecture/grounding to room.
Her punishments, looking back, were really over the top, and they were somehow about her, not about me. (Even at the time, I was always shocked by how strong her reactions were.) My choices, my "disobedience," and my consequent desperate lie were a sign, in her mind, that she was failing as a mother, and she couldn't stand that. So she punished, corrected, and expected cheerful, honest compliance from kids who were too cowed and skittish, too anxious and defensive, to be able to give her what she needed.
I'm giving a full a picture of what's wrong with that dynamic, not because I think you will necessarily be that way with your own kids, R., but so that you might get a sense of why you felt so angry about this normal childhood occurence. I hope you'll consider that alternate approaches to correction can be just as effective, and NOT corner your kids into feeling they have nothing but an impotent "I don't know" to offer in their own defense.
A far better approach is to observe what happened, and give your expectation that it be corrected, and give some time for that to happen. "Oh, no, I'm unhappy that your bins have been colored in. I expect that to be cleaned out within the hour. Come and ask if you need help figuring out how to clean them. If you want to decorate your room in the future, please talk to me first." And then be sure they have coloring materials, art pads, whatever, for their ongoing artwork.