Even if a Chinese speaker casually considers the leaves and the sky to be the same colour, it does not meant that s/he could not make the distinction if prompted. It would be similar to asking an English speaker to distinguish different shades of the same colour. For example, historically, English lacked a word for "orange" (which was borrowed from French) such that "red" and "yellow" covered greater spectrums respectively. Clearly, it wasn't that English speakers couldn't distinguish red, yellow, and orange because modern-day English speakers can do so perfectly.