Because it's in their eyes, it would have a visual interpretation. And some evolutionary advantage to that visual advantage, else the cells could be anywhere (unless there's a biochemical similarity to cones?)

However, a field isn't like photons hitting the retina. It would probably be a new colour, which we can't imagine, but I'm asking where that colour would appear, to indicate the field.

Perhaps a faint fog, varying in strength with direction - like the sky at dusk and dawn.