I think you can use blues, browns, yellows, greens, reds... As long as it is a nice collection of colors that is not to dark to make it male and you don't use to many pastels to make it female. Try mixing some blue with your yellow or some orange and tan? Avoid to much of the frills too, like flowers.
I agree. Use bold bright colors, in a nice mix of greens, blues, browns, reds, oranges, etc and it's pretty unisex. Here is a card I created last night:
It's leaning towards male, but if I would have made the journaling block in red, yellow or orange, I think it would go over great as a unisex card.
Kayson 10-9-06 & Cohen 02-08-10
i used some paper I had with bold colored polka dots of various sizes...tore that paper and had a plain color covering the other little bit of card. then used a cricut cut out of "its your day" from the george cart. (in a coordinating blue that matched some of the dots. Darn, I really need to take pics of my cards BEFORE I seal them inside an envelope!!! I said that the other day when I made future SIL's wedding shower card and what did I do??? Lick the envelope and then go "awww, I forgot to take a pic"