Giesemann 400W mogul bulb
 

I just fired up one of these (was supposed to replace 2 but unfortunately I was shipped a 250W instead of a 400W - it will be replaced next week)
It is a 400W 13000K megachrome E-40 bulb.
I have been running coralvue 15000K.
My intial observation - and as I only replaced one I have a direct comparison - is that this bulb is quite yellow. I don't know if this will change over the next couple of weeks as the bulb breaks in. I do supplement with PC actinics but you can still really see a difference from one side of the tank to the other. I will try and get some pictures posted either today or tomorrow.
I prefer (for my own visual pleasure) more of a blue look and actually prefer 20K bulbs. It will be interesting to see how my livestock responds to this spectrum. I don't have one of those do dads for measuring light pars or whatever they are called.

I have the 13K 250W HQI running on a Nova II pendant and don't find it yellow at all. It's not blue though, but definitely not yellow.

I took and posted a picture in my gallery that shows the contrast between the two sides of my tank. The giesmann bulb is on the right and to my eye it does have a yellow look. The bulb on the left is a coralvue 15000K

Thanks Muck. I don't know how to do move pictures.