I guess soil pH could play a role, but usually not enough to affect colors. Sometimes, the amount of sun a lily gets can change the color so if you planted in an area with more or less sun than where the lilies originally grew some variations could happen.

I think the main reason for the color change is probably related to the growth regulating chemicals used by the growers. We also purchased lilies that were bright orange, only to turn to a dark redish orange the following years. Growers spray lilies with growth regulators to keep them compact for shipping.

See picture below:

Here is a link that might be useful:

Because of intensive hybridizing, this may cause what you were seeing in your lilies--blooming orange the first year, and, reverting back to one or the other parent the following year. Thus, the reason you are now seeing a red lily.

Also, regarding the growth regulator Ken refers to often in his posts: many of the new varieties are available which do not need any growth regulators to control height of the plant. However, some of the older varieties still need a growth regulator. Using a growth regulator is not a practice we use growing lilies in unheated hoophouses on our flower farm. Our lilies are also grown for market.

Interesting..I hadn't considered much to what goes into hybridizing, just assumed it was a process of crossing one variety with another, and I can understand how they can revert. Thanks for your responses, I am always learning :)