[robski]

If I am not mistaken these 2 images were shot within 15 minutes of each other.

As I see it the auto-white balance is doing exactly what it was designed to do.

The human vision system is very good at adapting to different lighting conditions to workout the new white points. Because it is so good it makes it difficult to understand colour in the film and digital sensor world. Lets start with colour film. Films are made to produce a white for a given light source. Your Bog standard roll of film from your High Street supplier is termed as Daylight film ( typically 5400 - 6500 K). The sort of light you would expect at midday in summer. Pro films are balanced for studio lighting (tungsten or photo flood 2800K - 3400K) which contain less blue light. With Film you can purchase Lens filters to try and compensate for using one type for film for another light source.

With Digital sensors all cameras have an auto mode and at least a few custom modes which are hard coded to certain light sources in the same way as film. Higher spec cameras allow you more flexibility in setting the colour temperature of the light source.

In auto mode applying different skylight Lens filters is pretty pointless as the mode will try and compensate for the filter as well. The problem with auto mode is the camera has not got a clue of what you are about to photography. Therefore certain assumptions have to be built in. For the typical happy snapper scenes it does a reasonable job. I have found from taking measurements there tends to be a slight bias towards blue which I believe is to give you a better match for having photo lab prints made.

With the image in question the auto white balance saw the average scene as being reddish and corrected by reducing the red and increasing the blue channels. Based on reversing the auto mode correction using Clive's and Andy's method.

Attached you can see the average of Clive's and Andy's version (What Auto registered) is reddish mainly due to the warm sky (hint of sunset).
Now compare that with the average of the original posted version.(Auto Corrected) it has made the average a gray with a hint of blue.

In very simple terms auto mode assumes that your scene will average to a gray and will adjust the channels until it is.
If your scene contains a wide gamut of colours it works fairly well. However if your scene has one or two dominate colours it make a pretty bad job of it because the average of these colours is never gray and the auto mode has tried to make it so.

So it is down to correcting at the Post Process stage at outlined by Clive. If colour is important to you take a mental note of what reference points (true whites and grays) are in your shot.

To go a stage further don't use auto mode but select a mode that best suites the lighting at the time. Also carry a gray or white card reference around with you and photography it under the same lighting conditions to get a gauge for the amount of post processing correction required.

A lot of bother I know and thats why most of are lazy and use auto mode.

Andy I can see that physics is not your strong point.

Light at the beginning and end of day by nature of it's low angle has to travel through more atmosphere than at midday. Which has the effect of attenuating the higher frequency blue light more that the lower frequency red light.Therefore the light at these times contains less blue light ( seen as warm red/yellow ) than at midday.

Light source Colour temperature can be very confusing. Traditionally we tend to say that reds are warm and blues are cool based on the mood these colours put us in. From a science point of view red is cool and blue is hot. Colour temperature is based on heating a black body mass and seeing what colour light is emitted at different temperatures in degrees kelvin. Red Light has low energy levels 2500K and Blue has a high energy level 7500K. Amazing to think that shade in blue skylight has a colour temperature of 7500K and Blue sky can be 10000K plus.

When it comes to Post Processing colour correction using Photoshop RAW processing for example. At low temperatures correction you are adding blue and removing red to compensate for the yellow whites. Conversely at higher temperature correction you are removing blue and adding red to compensate for the blue whites. The green channel remains fairly constant. If you have a tint adjustment that adjusts both the red and blue in the same direction giving the visual effect of adding or removing green.