I've got two displays. One is approximately six years old: Fujitsu P24-5 ECO (LCD/IPS, CCFL backlight), the other one is a DELL U2412M (LCD/IPS, LED backlight).
I've tried to color calibrate both displays (LinuxMint, dispcalGUI, 6500K, 140cdm²) with three different colorimeters that I could access (an old-ish Pantone Huey Pro, a brand-new Colormunki Smile and my very own brand-new Colormunki Display).
With all three colorimeters (and a handfull of different settings concerning the profile type etc, including color correction matrices for the Dell Display with the Colormunki Display - couldn't find correction matrices for the Fujitsu) the results were essentially the same: Checking with the eye the white point seemed to be very close for both displays, as were brightness etc. But on closer Inspection the Fujitsu does have a slight red-ish tint (compared to the DELL, that doesn't yet imply which one is closer to the "real" colour). This is almost not noticable (i. e. only with side-by-side comparison) for sunsets or brown leafs, but it's a big difference for lighter skin tones (giving someone a sunburn only on the Fujitsu Display).
On yet closer inspection the TRC curves of the display profiles (that I can view from the colour panel in my system settings) look a bit different:
The Dell TRC curve has all three colors of RGB neatly on top of each other.
The Fujitsu TRC curve seems to indicate that red is somehow intensified.
That said, I don't know the real meaning of this curve. Especially I don't under stand why, if its meaning should be "right, the Fujitsu is tinted red" why this wasn't compensated through the display. Or could it be the other way round: Red is intensified by the profile because the instrument read a smaller brightness for red and wants to compensate for that.
But then, if the latter should be the case: I did calibrate the displays with the colorimeters before each profiling, so the colors brightnesses at maximum brightness should have been essentially the same to begin with - at least that was the colorimeter readout during calibration.
Could it be, that the instruments read the three colors differently during calibration vs. profiling?
I really don't understand what's happening here. My expectation would have been two essentially equal color reproductions on both displays.
Would it help to buy a spectrometer (e.g. Colormunki Photo)? Or what else could I do?
EDIT: MirekE suggested in an answer to try and set the standard observer on dispcal (-Q 1964_10). Unfortunately this doesn't change much and the problem still exists.