Hearts are a universal symbol, and as such, certainly for me, they speak volumes in any piece of work. Sure, they are open to interpretation, but even so the base message is always prominent.
Why is one heart orange? What does that signify? Actually in this piece it doesn't mean anything.
It was coloured as such to break up the block of red from the adjoining hearts and also acts as an anchor in a triangulation of the other yellow/orange elements in the painting. It also contrasts well with the larger blue passage. It has been used as merely a colour device.
Sometimes a composition will dictate requisite colours and tones.