1876 - 1879 : EFO Missing Color Or Just Faded Stamps?

Found these in a batch of postage. The pink background seems to be missing. It's very uniform, doesn't really look like simple fading... Does anyone know if a variety with no pink background exists for Scott 1876-1879 flower stamps?

When placed under a strong light, this issue will lose color and even some of the tagging. This is based on my own experimentation as I was asked to verify a color missing on this issue and reproduced the "color missing" by exposing a block to direct lamp light for two days. For proof I covered one half of the block and exposed the other half to direct light.

wtcrowe: Was it incandescent light? I might try to repeat the experiment.

Lightfastness can be tested in many ways, using an incandescent light would probably work but might take longer. UV light would be the fastest, a bright southern facing windows would probably get results in a day or two. There are also many international standards applicable to lightfastness testing (ISO 105-B02, BS EN 20105-B02, BS EN 4892-2 and ISO 12040); you can Google any of these and/or 'lightfastness testing' and see how the pros do it.
Don.