Palomino Stallion + Flaxen Chestnut Mare = ??? Foal

What colour might the foal be?

50% Chestnut
50% Palomino.

The calculator doesn't have a flaxen option, so it's probably something more like:
25% Chestnut
25% Flaxen Chestnut
50% Palomino

As Poseidon said, 50% chestnut, 50% palomino.

As for flaxen, it isn't understood yet how it inherits, so there is a chance that if the foal is chestnut it might be flaxen. Anything beyond that *shrug* who knows lol.