Colour Temperature and Colour Balance
In order to achieve good color balance with studio flash, the user must understand the factors that affect the colour.
Cameras with built-in flash need only establish color balance parameters from a single, unchanging flash source and are normally set to a "flash" setting for proper colour.
But color balance with studio flash is affected by a number of variables, including the color balance of the flash units themselves, changes of color introduced by the shooting environment, changes caused by light modifiers and the artistic perception of how colours should look in a shot.
The first rule of seasoned professionals is that there is no reliable camera setting that considers all of these factors. The assumption that all lights rated at a certain colour temperature (such as "5600K") produce the same colour balance is incorrect. In particular, the "flash" setting of most cameras is not accurately matched to most studio flash systems. There is simply no automatic method of setting accurate colour balance in the camera. Fortunately, there are two methods of easily achieving good color balance.
Custom White Balance
The proper method to do this in the camera is to follow the camera manual's instructions for setting a Custom White Balance. When you do this, a test picture is taken with the lights set up and adjusted for the images you are about to take. The camera then calculates the appropriate adjustments needed to render neutral colours (greys and whites) taken in this lighting environment free of colour casts and errors. Once this is done, the entire colour spectrum will be reproduced as accurately as the particular camera allows for. Using custom white balance is the preferred method of colour balancing if you are shooting jpg pictures or when you wish to avoid post-processing of your shots.
With the evolution of software programs such as Adobe Photoshop, most professionals prefer to shoot their essential sessions with the camera set to "RAW" mode rather than jpg. This method saves the information directly from the camera sensor to the memory card, bypassing any processing in the camera. Thus, you can shoot spontaneously and avoid all colour balance, contrast and sharpness settings in the camera with the assurance you can more accurately set these parameters after the fact, in post-processing.
A word of warning: Shooting in RAW cannot undo burned out whites, out-of-focus shots or improper ISO and shutter speeds settings from the camera and thus requires you to set these parameters properly in the camera.
The proper method for shooting RAW is to ensure that you have a known-neutral object in the scene. If you are shooting a series of pictures under the same illumination, you can place a white or grey white-balance test card in the first shot of a series, then remove it for subsequent shots. Many shooters prefer to leave a white balance card at the top or bottom of every shot where it can be cropped out later, as this gives them the freedom to make significant changes in the lighting with the assurance each image can be critically colour-balanced in post-processing. If you are shooting against a neutral grey or white background, you can often omit the white balance card and use the background as a neutral object.
But beware that all grey or white backgrounds are not genuinely neutral and that filters, gels, etc., must not tint the light falling on the background. The procedure for post-processing RAW shots is quite simple. In Adobe Bridge (part of Photoshop CS2 and CS3), you can open your entire pictures folder. Select a picture that contains a white balance card and place the eyedropper tool on the white or neutral grey portion of the card. The software will instantly calculate the perfect colour balance for the entire image. From there, you can adjust contrast, exposure, cropping, sharpness and other attributes of the shot.
If you have shot a series of pictures under the same illumination, you can copy and paste any or all of these parameters to all the other shots in the series. In RAW processing, the original image data is usually saved to the hard drive. A copy of the image containing your adjustments is output to a new file as jpg, tiff, Photoshop or other file types. If you are later unhappy with your post-processing, it's easy to reopen the image in RAW, apply new settings, and output new final files. Smart shooters always archive their original RAW shots.
Does the use of various accessories affect the color balance?
It is quite possible for softboxes, brolly boxes and umbrellas (and other fabric or paint-based accessories) to produce either warmer or cooler colour temperatures, even from batch to batch. This is typical with most all fabric-based accessories - the vendor buys the fabric, and the fabric supplier gets it from different batches. It's pretty hard to control - even for the most expensive brands.
The same is true with basic flash colour temperature and cameras. 5600K might be neutral on one camera and 300K-400K off on another camera. There is also a green/red component of colour balance that is not stated in the basic colour temperature specs, so even an exact match in colour temperature alone will not yield a perfectly neutral colour balance. There is a lot of controversy about this subject, and the consensus is that one cannot rely on color specs or camera settings to achieve perfect color balance - with any lights or accessories.
The consensus also is that all accessories will alter colour balance by up to several hundred degrees. The only way to properly colour balance is to do a custom white balance in the camera or, preferably, shoot in RAW and colour balance in Photoshop or another RAW program.