The mistake most photographers make is to think of colour in terms of absolute values and "correct" colour. Both assumptions will lead to you mis-understanding colour and how it works.
Colour is not a property of light but how the eye "sees" different wavelengths of light, and the eye does not see in absolute values but only in relative values. It is only in comparison that we see a
relative difference. For instance SkipRD only sees the colour cast when comparing the two images and not in the one image alone. Even when you learn to look at an image and can see that maybe it looks too blue it is mainly because you're comparing it to a memory of how you think it should look.
Perception and Memory
Sabrina is quite correct here. Many photographers talk of differing perceptions between people as though it's an excuse that allows them not to bother learning about colour, "your perception may be a little different to mine."
If you and I were to match colour swatches side by side, (assuming no defects in vision), and were then to compare our matched swatches side by side we would both agree that they were identical. It does not matter, (and impossible to prove), if my blue is your red what matters is that if I colour balance a photo in direct comparison to the original scene my version would look correct to you just as your version would look correct to me. They would be consistent. To all intensive purposes colour perception, (the way the eye physically sees colour), is assumed to be the same for everybody.
However if you and I were to match colour swatches by looking at a colour then immediately finding a match based on memory alone you would find that virtually none of matched swatches would be identical.
We do not perceive colour differently but our memory of what we think we just saw can vary tremendously.
Glancing and looking
There are two other processes at work and by far the least understood is the difference between glancing and looking, again I agree with Sabrina in that you have to train your eye to look rather than glance.
With the assumption that vision is absolute, and because the camera captures absolute values, comes the assumption that suddenly a print or image on a monitor can show absolute and real colour. But this is absolute nonsense, (that is the only absolute thing about it ;-) ).
What you see in your mind is not a direct connection to the eye but a construct of the brain. The eye is not a very good optical instrument, but it has amazing resolution and centre sharpness. What you see is really a result of your eye scanning the scene and your brain piecing it all together as a coherent whole. With the resolution of the eye this can take a small but measurable amount of time to process.
The thing is that if you just glance then you do not give the eye or brain time to scan and present you with a complete picture. In such cases what you think you see as absolute is in fact your memory filling in the gaps. As much as 90% of what you thought you just saw can be your brain presenting you with a view of what you expect to see.
Do not underestimate your brain's ability to correct and alter what the eye sees so it fits in with your logical expectations.
A little knowledge is a dangerous thing
Perhaps the most difficult thing to understand and believe is that once you create a framework and a logic to how you think things should be you then become effectively blind to anything beyond it.
I'll try and give an example to illustrate. You're in a gallery and making a technical copy of a Monet. Very cleverly you light the image so as not to get any specular reflections, you expose and capture the information perfectly. You then get it back to your monitor correct the WB and set your black and white points.
Hang on, is there an assumption in there? You're processing to the logic you understand and what you assume to be correct and in doing so have completely failed to look at and understand what you're photographing. Monet seldom used a black point, his omission of it was deliberate. How many of you would have thought to even look and check this and how many would just automatically follow a process they assume is correct? How does this lack of a black point affect the colour in the images? How would you know if you've never actually noticed it and never look for it? You're effectively blind to any process outside your own because you only see images in reference to the processes you understand, and with it you loose a wealth of information about how Monet understood and used colour.
The same with a real landscape, if you just process to what you assume to be correct then you fail to look and understand how colour really works and how you can vary yourapproach to produce different moods and effects.
Colour is relative and not absolute
An old trick? Optical illusion? No it is in fact a very good indication of how your eye actually sees colour. If we look at the image below which side looks more yellow?
They are in fact both exactly the same. Now to go one step further, by changing the luminosity of the yellow I get the following. Now open both up in separate tabs on your browser and flick between them. Which side changes the most?
Again the second image has exactly the same yellow on both sides, so why does the yellow on the right appear to change more?
It does this because our perception of colour is largely relative. Though you'll never stop a bright yellow being yellow, how bright/dark or vibrant/dull it looks is largely influenced by the colour it is shown with and not it's absolute quality. Many think of colour as absolute in that if they want to make something more yellow then they add more yellow and turn up the saturation. They then completely fail to see that they have actually made it look a lot duller than before, (because they are effectively blind to anything beyond the process they assume to be correct).
If you want yellow darker then show it against white, lighter then show it against black, and if you want it more yellow then contrast it against blue.
The yellow on the right appears to change more because the interval between it and it's ground changes more. The yellow is no longer an absolute value or colour. It not only changes depending on the colours it's shown against but how much it appears to change is also dependant on the interval between the colours it's shown against. Even on the same image the same colour can look different. What is demonstrated here is that the appearance of colour does not have an absolute numerical value, that because we see it as different does not mean it has a different absolute value and that how colourful something looks depends on the way the different colours interact and not their absolute values.
So how does this affect WB?
Easy, the best WB is the balance of colours that gives you the best contrast or most colour, the point at which the variation of colour works best, (a colour cast adds the same hue to everything and so reduces variation, contrast and the appearance of colour). It is the one that's closer to what the eye expects to see so the eye does not correct it so much when viewers only glance, and because we do not all expect to see the same then you can vary it a little by making it slightly warmer or cooler. You could also increase the interval between the warmer and cooler colours to give more punch without reaching for the saturation slider. ;-)
Above all it's not the one that corresponds to a specific number or absolute value.
The far right is your original, the far left is a more real rendition for a mid-day sun. But a real rendition is normally quite un-rewarding. The middle is a slightly warmer touch. Now you may have thought at first that there was no difference between the far left and the middle, if you only glanced, but look a little closer...
