Wow, isn't that odd. All I can imagine is that sometime there were some roots in the ground that were brought up when you prepared the soil. Tiger daylilies don't have seed that I know of, just very agressive roots. I'm sure it is possible for your yellow & pink daylilies to create orange babies, but not likely they would come out like tiger lilies. Maybe you have a "helper" comeing in to plant for you in the middle of the night? Just to see if they can drive you crazy :) It is really hard to get rid of them, but not impossible. I think digging is the best answer, just keep at it. I think they are resistant to roundup, at least for me.

I went to a daylily farm once and bought some plants. I bought two fans of a nice cream colored tetraploid and when it bloomed, one fan had a nice creamy colored flower. The second fan had the orange flower of one of the tiger, ditch or whatever you call the 'wild' daylilies.

Apparently, the daylily farm had them growing close to the other daylilies. They propagate by underground rhizome, and had sent a fan up beside my creamy tetraploid. I dug the whole thing up, carefully separated them and sifted through the dirt to make sure no rhizomes had escaped me. I replanted them in separate places.

Probably the easiest way for you to get rid of them is to dig everything out of the area of your daylilies, at least 6 inches deep and at least 3 feet adjacent. Put a root barrier around the plot you want to plant the tetraploid daylilies in and fill with fresh soil that you are sure is free of rhizomes. Carefully clean the dirt off your tetraploids and replant them. Spread the old soil out and watch it for new daylilies sprouting from the old tiger daylilies or their rhizomes. Dig those out while they are small and dispose of.
Good luck,
Ed