Isabelle walked around the room to address the questions being asked of her - it seemed kinder after her shouting outburst, which he did feel a little bit guilty about. A little bit. But it had to be done.
"It's slightly more complicated than changing your hair, I'm afraid," she said, standing next to the Slytherin who had spoken.
"I don't want any of you to try and get it wrong, thus damaging your eyes. Healer Brookes might not be happy with me. Or your parents for that matter."
Good question about criminals, actually. Smart kid.
"They fade, certainly the colour changing one does. But you need to use the counter-spell to stop them completely - Intaris. It's the same for both, so hopefully you'll all remember it." Isabelle gave the girl a nod and smile.
"And as this only changes hair...and sometimes eyes....it's not really enough for a criminal to become significantly different. Unfortunately, there are other spells they could use for that if they really wanted to." Not that she was going to say today. Just in case she had potential criminals in the room. Heh.
Oooh, Ellie had a question. Pretty decent one, too.
"Generally people use it to change the whole head of hair, it is easier that way. However, with everything Transfiguration related, concentration and intent is key. So if you wished to just change part of your hair, it would require more effort, but could be done, yes." "I believe there are some books on the subject, yes, which I'm sure would be in the library here. I'll check my book list when the lesson is over and let you know," Isabelle smiled at the young Hufflepuff. It was nice she wanted to learn about it.
But now was the time to get on with this lesson.
Unfolding her arms, Issy walked back to the front of class and changed the words on the board.
"Now you will note that the way you perform this spell is the same as the previous one. But the outcome is very different. This only changes colour, not length or style - which hopefully you have all already done." Yes, no? Scanning the room, she couldn't really tell, but most of them did have strange hairdos.
Holding her wand to the top of her head again, Issy tapped once and said the spell clearly so they could hear.
"Ris-TAR-is!" Her hair changed from it's usual dark brown to a brightish blonde. Similar to the way she had changed it a couple of terms before for the Prefect's tea party.
"You have to add extra emphasis on the middle portion of the word for it to work properly.
Smiling, she re-holstered her wand and leant against her desk.
"I don't expect you to all turn it red like I did before. I was having fun with that - clown hair can be any colour you like. Whatever you feel fits with your make up and costume. And you CAN do it in pairs if you so wish. Getting a colour wrong is nothing that can't be fixed, unlike length and style change, which is why I wanted you to do that to yourselves."
Isabelle fiddled with her newly blonde hair, before flopping her hands into her lap.
"Please begin, and if you need any help at all, just put your hand in the air and call me over." OOC: I'll be off to bed soon, so probably won't post again tonight. Isabelle is always there though - but PLEASE listen when a Prefect asks you to do something. 
Any questions, or help required, will be dealt with tomorrow. Have fun choosing your colours!