Originally posted by kuau I think the 645d likes grey for setting custom WB as opposed the the expodisc which is white.
This maybe your issue.
You may want to call Pentax to verify
You might be on too something here. My collapsible, fabric grey card is grey one side, white the other - if I try using the white side the WB is terrible.
I was caught out some time ago when I only shot jpg when I first switched to digital from film and I botched a shoot due to WB issues. Checking WB prior to taking that first shot is now an automatic thing for me and used to spend a lot of time making sure it was spot on.
Now with the 645D I'm trusting the default settings like daylight, tungsen, shade ...etc and well as the auto setting a lot more - I find the camera really does do a good job of it and if it is off it's very easily fixed in ACR.