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I have a new 27" iMac, on which I run Fedora 15 (with no Apple OS) — just boot straight to Linux.

The colors are basically accurate to my eye, but the white point is much higher than I'd like (that is, everything is quite blue). I suppose it's 9300K, and may be due to fancy LED backlighting or some such. I dunno; I'm a software guy.

I'd like to adjust it to a more comfortable 6500K. In the olden days (including my old LCD monitor), one would just push a button on the monitor and select that from the menu. On an iMac, of course, that's not really an option. Can I do this in software instead?

I know that full-on profiling and calibration software runs on Linux, and I may get to that eventually, but for now I'd just like a simple option. What should I do?

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If you can source an appropriate ICC profile (or hack one that is close to what you need), you can use xcalib to run it from your .xinitrc xcalib.sourceforge.net – jasonwryan Aug 2 '11 at 4:04
@jasonwryan: I found a couple of ICC profiles on various web sites which say that for an iMac model and vintage which seems similar to mine, but they all give horrible results. I can't find anything official (but maybe I'm not looking in the right place). – mattdm♦ Aug 2 '11 at 4:12
I have only found this tftcentral.co.uk/articles/icc_profiles.htm#the_database You might find something to work from there... (I'd be surprised if you found anything official - it's not exactly the Apple Way) – jasonwryan Aug 2 '11 at 4:23
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Related interesting software redshift. – Stéphane Gimenez Aug 2 '11 at 12:27
@jasonwryan: unfortunately, that's one of the ones I found that isn't right at all. (It appears to be for a 2009 iMac model with a different screen.) – mattdm♦ Aug 2 '11 at 16:46
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Okay, so, the answer is that the redshift program, pointed out by Stéphane Gimenez in a comment above, can do this pretty simply. It's meant to do a clever adjust-white-point-over-the-day thing, but it can also be made to do one-time adjustments with the -o flag.

The desired color temperature in Kelvin must be given twice because of the time-based features, which is a bit weird but unimportant. And the other stuff is just there to suppress unhelpful-in-this-case informational messages — it's not important.

 redshift -o -l 0:0 -m randr -t 6000:6000

It's also important to note that the code assumes that neutral is 6500. (And experimentally, -t 6500:6500 appears to me to be identical to using -x to remove the effects completely.) On my system, where I think the native whitepoint is 9300K or so, I'm not sure what the actual numbers end up meaning, but the effect of choosing something lower than 6500 is certainly exactly what I want.

I was curious, so I looked at the code. Redshift is doing something a bit more complicated than running xgamma — it has a whole color-ramp table, with data sourced from http://www.vendian.org/mncharity/dir3/blackbody/. I'd look into it even further, but I think the next step is going for full-on proper color calibration.

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Latest version allows a simple redshift -O 6000. – grawity Nov 3 '12 at 0:15

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