Easiest way would be to divide the castle in half (right down the middle).
Boy half would be gray with dark gnarly vines. Could be accented with shields, swords, ravens, knights, etc.
Girl half would be white with trailing vines and dainty pastel flowers. Could be accented with fairies, princesses, pretty little birds, etc.
HTH
Ok, well if it was me then I would do something similar to LINDY SMITH, I think the best way to go, would be to the castle in a neutral cream colour, like she did in hers, and I would do a dragon wrapped around the bottom in either oranges and reds or go for blue's....then I would do in the window a princess with long hair and a tiara, so it looks like the dragon is guarding the princess, and that way they fit together and wont loook out of place you can always add more to it if you want a few vines with flowers and even some golden treasure around where the dragon is. Hope this helps I think that this would be the easiest and both the boy and girl would have some thing.
goodluck, and what ever design you choose it would be great to see some pictures 
Hi, me again ive found a link that will a show you a picture that might help you, this dragon in the picture by debbie brown is fairly simple and quick to make you just have to imagine the dragon being a bit smaller, and then a higher tower which is where you can put the princess in. put the colour of the castle is the same to what i thought...but you dont have to do it that colour, it was just a sugestion because it was neutral. goodluck
http://www.debbiebrownscakes.co.uk/magical_cakes/pages/dragon%20castle_jpg.htm
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