When an image is all white it means the entire scene is overexposed to the point all three color channels are fully saturated.

Think of it this way: If a scene has twice as much blue as red and green when properly exposed the amount of blue recorded will be twice the amount of red and green recorded. It also means it is possible to fully saturate the blue without saturating the red and green. But if the red and green are saturated then the blue will also be saturated because that would mean the blue was exposed twice as bright as what was needed to fully saturate the blue channel.

With a digital sensor (or even with film) it is not possible to increase the level of a color past saturation: that is the highest value for that color recordable by the sensor or film. If you just expose barely long enough for the entire photo to be pure white (full saturation in all three color channels) or if you expose for ten times that long the photo will be equally white. The one exposed for ten times longer won't be ten times as bright because the shorter exposure also let enough light strike the sensor to fill all the pixel wells until they were full.

It's like a rain bucket. If the rain bucket is only big enough to hold two inches of rain and it rains four inches, the bucket won't be any fuller than when it was overflowing after the first two inches had already filled it up. If a bucket that can only hold 2 inches on one side of the yard got 2 inches and a same size bucket bucket on the other side of the yard had four inches of rain fall on it, they'll both be equally full. We'll have no way of knowing how much more rain might have fallen on either side of the yard.

In order for an image to contain usable information (which is what we would call a properly exposed photo), some of the "buckets" (i.e. pixel wells) have to be fuller than others. If they're all totally full then there is no difference between any part of the image and there is no usable information.

To reduce exposure you have several choices:

• Reduce the camera's sensitivity by setting it to the lowest ISO setting.
• Reduce the amount of light entering the camera by using a narrower aperture setting (higher f-number).
• Reduce the amount of time the light is entering the camera by using a shorter shutter time. Each time you halve the amount of time the shutter is open you halve the amount of light that strikes the sensor. If an image is pure white it usually means you need to halve the shutter time at least three times (i.e. three "stops"). For example, instead of 2 minutes (120 seconds), try 15 seconds. Or instead of 1/30 second try 1/250 second.
• Reduce the amount of light striking the front of the lens by using a neutral density filter. They are available in various strengths from one stop (half the light) to ten stops (1/1024 the light). Avoid cheap "variable density" filters if possible. They cause a lot of image quality problems and color shifts.