Blue... lol!
There is SO much information on the net, let alone here about this topic... but to answer your question.
Going higher than 35W in the stock FOG housing is probably NOT a good idea, if you have doubts, go for it, disregard the numerous postings about the housings melting after a time using 55 watters. BEFORE I did mine, I read all about it and decided to get the BlazerTech fog's (35w and yes, yellow) mainly due to the housing being METAL and an H3 lamp that if I wanted to at some time in the future upgrade it to 55w, I could without concern.
The main headlamp assembly I'm not so sure there's as much worry, as they're all plastic that I've seen, so shot the moon here. I've also read again and again that using 55W HID's in the STOCK housing will cast way too much GLARE and cause problems while on the road and even tickets! But if you're one of those who don't give a %$^# about other drivers (high beams = 100%) then go for it. The only thing I WOULD recommend is that which ever you decide, even 35W Fog's, is that you upgrade the wiring as it is not designed to handle the increased amperage the lamps will draw from the circuit when installed. Sure, it'll work without it, but for how long is anyone's guess and do you really want to burn up some wiring under the hood? Hmmmm... IF I could suggest anything, it would be to put your money towards a RETROFIT job on the lights if you are not happy with output. This way not only do you increase the output, but you actually get to put some light where you want it... on the road!
Good LucK!