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As the chart annismyrph linked clarifies, there is also a combination of white spotting patterns that when combined, will result in a foal that is not viable. Frame does not necessarily need to be present for a lethal combination to occur!

There are 3 possible causes for a foal to not survive from excessive white in the game. The announcement of the dead foal is the same no matter which was responsible.
The chart annismyrph posted shows the probability of a lethal foal from combinations of kit mutation genes.
True LWO, is called Frame (for the spotting pattern of heterozygous horses), and always lethal when homozygous (except for the rank special Freaky Friday, if GMTed or if on the second gene set of a chimera).
Splash 2 and Splash 3 are also always lethal when homozygous or if combined with each other.

It's because pigment is a vital part of nerve development. It is the same reason why white markings in the ears can so often create deaf animals. Pigment is very important.