Measuring color temperature.
The Colormeter II will give you a Kelvin readout and advise you on
the filters you should use to compensate. It's for ambient light
however and to use it with strobes you'll need the additional
It's not that accurate though. I used to use it with slides. If I
measure on different days and follow the advise of the meter I get
a distinct difference in color.
Much more accurate and cheaper is to use a graycard (or something
like that) and C.T. filters. These are lampfilters that convert
daylight to tungsten and vice versa. You can get these in Blue and
Orange in 1/8,1/4,1/2 and 1/1. Take a photo and measure the color
in Nikon Capture or Photoshop. Put filters over the flash and
measure again. You can adjust it for values in between the filters
by covering only part of the flash with a small piece. This way you
can very accurately adjust the color of every flash or diffuser to
the exact color temperature you use in the camera.
I agree with FotoFlip.
You do not really need to know what the absolute Kelvin values are, only what comparative effect they have on a photograph. So taking test photographs is the logical and more accurate way to find out. Also, by using the actual correction filters as he suggests, you will be going directly to what works.
Small samples of the CTB filters mentioned above are available in a swatch book, usually supplied free of charge by makers like LEE and ROSCO. All the available filters are in there, [a vast range] in a size just about adequate to cover the face of a small flashgun --- so maybe that's what you should use as test source light.
Every photographer should have his own copy of the filter swatch books -- maybe two if he can get them. One to cut up, and one to keep.
If your diffusers are so yellow as to be problem, it is usually a sign that they are getting old and need replacement.
It is perfectly possible to bring your lights back to a common balance (being the SAME colour is the most important thing, of course,) by orange filtering the lights which are NOT shining through diffusers. For this you will need CTO filters -- they are available in the same gradations, and are in the same swatch books.
Filtering the non-diffused lights might be the cheaper option. If they are smaller you will not need buy so much material. Also, filter gels fade, and blue ones fade faster than orange ones!
Hope this helps.