What Did princesses where in the 1400's?

If you are meant to be a tomboy, perhaps they expect you to dress as a boy? That would mean a tunic, about down to your knees, with some thick tights (pantyhose) underneath. The tunic should preferably be in a colour that is not too jarringly modern - blue, green, red and yellow were all popular colours in medieval times. Some short boots, or some slippers, would be the best footwear, they would look more medieval than ordinary modern shoes. And maybe a belt with a dagger in it (a paperknife would do, or perhaps you could buy a toy dagger, most toyshops have toy weapons of various kinds). A short cloak perhaps if you can manage it (a couple of yards of cloth - velvet perhaps, or something that looks like woollen cloth, flung round your shoulders, will look good, and you could pin it at the shoulder with a brooch).