Remember, Aku cannot change his color scheme when he shape shifts. He doesn't have any purple at all on him (Only Green, Red and Black). If you go back a few pages to when he made Chi laugh, you can see that he is still holding true to this rule.
Here is a quote from the page on Aku (http://samuraijack.wikia.com/wiki/Aku
"Shapeshifting: Calling himself the "Shape Shifting Master of Darkness" at the opening of each episode, Aku can change into virtually any form he desires, from a giant dragon to a small insect, or even a beautiful woman. He can also change his size, liquefy himself, stretch, and expand his body. Aku's consciousness can exist even in a part of himself. He typically changes into animal forms that are threatening to fit certain situations, such as a scorpion or an octopus, adapting to the situation. He is however limited to his own colour scheme, no matter what form he takes, it will always be black, red and green. Also, his true reflection will always appear, even in another form."
Edit: Also, I think that the alarms on Castle Grim would have went off if Aku/Chi had somehow managed to infiltrate it (Remember what happened in "Castle Crashers" when the security cameras caught Mimi?). Not to mention how angry Mandy is at being blindsided twice. I would hope that she increased the defense around and inside of Castle Grimskull.