Green structure

This strange green building appeared without warning or fore-notice beyond St Matthew's in the Short Strand. Initially it appeared light green in colour but before our eyes, with no perceptible change in illumination, it became darker in tone. Green thing, lightGreen thing, dark

Update: from looking at a map this may be located in Dr Pitt Memorial Park on the Newtownards Road, which would explain its sudden emergence and temporary appearance. Its strange tone-shifting capabilities remain a mystery.

Update 2: Even more bizarrely, if you look at the tone of the trees behind the structure, they are always the inverse of the building's tone. I wonder if it is an optical effect.