Change color key
I have an older laptop, an HP 6715s which has a warm white CCFL display. It is just awesome, never gives me eyestrain even in dark or poor lit environmens. However, I also have a Dell E5440 which is my work laptop and sometimes I bring it home and work on it in the evenings. I use F.lux because it has a LED-backlit display and the amount of blue just kills my eyes within an hour when I'm in a halogen-lit or a poorly lit environment.
However, I don't really like F.lux's orange/amber color shifting either. It looks very unnatural as no display (even warm white displays) would use a color tone / temperature like this. If I set it to around 4300K not to have too much amount of warm tone, the white surfaces on the screen will have a pinkish / even purple tone. This is the amount of blue light coming from the backlight is mixing up with the little amount of amber/orange tone F.lux brings in.
So I would like to adjust the color key of F.lux and change this amber/orange color towards what F.lux shifts the color space of the system. I would like to have my LED-backlit dell to have the color temperature of my HP 6715s. However, its color tone is far from F.lux. It's kind of yellowish, a real warm white CCFL.
How can I do this with F.lux?
No, you have to calibrate your display. F.lux isn't the problem, it's the color calibration of your monitor.
If it could be done with color calibration, I wouldn't even use F.lux at all. I'm okay with warm white in daytime as well. However, you can't do this with a laptop display, using only system configuration tools. You can't change the color temperature of the display.
I'm sure F.lux has a built-in color key toward which it shifts the entire color space. It's about the same color you see in the upper part of the F.lux logo.
All I'm saying is, the color accuracy of the monitor dictates what you see. So, you need a monitor calibration device to calibrate the color of the monitor to have more accurate colors so that it's easier to get what you want out of f.lux. That's all I'm saying.
I had to deal with this problem with the 8-year old 22" Samsung 2253BW gaming monitor. It's a great monitor for gaming, but it has horrible color accuracy: it has a very blue washed-out image. Calibrating it for greater color accuracy resulted in a MUCH better time using f.lux. I was far less frustrated with it. I was finally getting something much closer to what I was expecting. Y'know what I mean? If you have two different monitors (regardless if it's a built-in monitor on a laptop or not) and if you have two different color accuracies, then of course you will have two different experiences with f.lux. The solution is, use a monitor calibration device. Yes, that costs money, but that's what you will have to do. Or, you can just deal with it because seriously,. there's nothing else you can do outside of trying to manually adjust colors yourself which is a big pain in the ass.
Now, I was lucky in that I didn't have to buy a calibrator. I found a properly-calibrated profile for my monitor. I just looked for it, found it, downloaded it, applied it, and bam: I had much better color accuracy and thus a far better experience with f.lux. If it weren't for that, then I would've had to buy a calibrator, but I would've had the same end result.
That's what you're dealing with, just as you said. You said the color accuracy of that panel isn't good, so you need to make it better. If you don't do that, then you will never be able to improve your experience with f.lux due to the way f.lux works.
You are right about color calibration but it's not what I'm talking about.
F.lux uses an amber color to overlay the whole screen proportionally to the color temperature you set on the scale. On all devices - regardless of color accuracy - it is the same overlay color. This is what i call the color key. It must be changed. However, today I already tried Redshift which gave me much better results, than F.lux.
It's not an overlay. That's the thing. Redshift uses an overlay (it's a "cheap" way to do it), and it reduces the contrast and it's not as good as you think it is. It's not as true as f.lux, and I know you disagree based on your experience, but I'm telling you that you need to calibrate your monitor. I know from experience. That's all this is.
Then it is some playing round with RGB balance. Same effect. There is a color toward which the white balance is shifted. So I would like to tell F.lux how to shift it.
@TwoCables What do you think about the color accuracy of my CCFL display in my HP 6715s? CCFL displays have the worst color accuracy because CCFL backlight has a poor spectrum. I don't care about color accuracy. I care about white balance. And F.lux is lacking a basic feature to adjust which color the white balance it shifts to. This is my problem. I'm not going to buy expensive hardware just because software developers were too ignorant to add a basic feature (IMHO). I don't need color calibration, at all.
No, Redshift is not cheap. It gives a way better experience and there is no pinkish, purplish white areas around 4000-5000K. If I want to do a color sensitive work I will turn off Redshift. As I would turn off F.lux as well.
At warmer color temperatures f.lux usually tries to display colors on the "Planckian locus" (the "blackbody curve") and so there isn't one tone as you describe. However there are a lot of applications for using whitepoints that are not on this locus so we have spent some time thinking about that and will ship it somehow in the future. One question is if your HP is close to a blackbody or not. It would be fun to measure how a CCFL degrades over time!
Overall, screens that look really bad ("purple" or "pink"?) are poorly calibrated, so I think @TwoCables may be right - you are probably describing a screen with bad calibration.
Then, I think you are mostly suggesting that we should let you do the calibration "by eye" instead of using a device that's more objective. But it is important to say if you mean, "my screen displays accurate color and I want this off-Planckian color" vs. "my screen has a terrible calibration and I want you to fix it".
Technically, we can match any chromaticity at all, but I really prefer to do this with an actual colorimeter instead of using sliders....it is just very hard to do it by eye.
Also, our FAQ links to https://quickgamma.de/indexen.html as it lets you do this in a way that's compatible with f.lux, so I would start there. If you want to do this in a better way, I really like x-rite's colorimeters.