皎洁 is almost always paired with 月光 (moonlight) or 月亮 (moon) — it's a literary, poetic word and rarely describes anything else. For ordinary brightness use 明亮 (míngliàng); for white objects use 洁白 (jiébái).
memory
皎 contains 白 (white) on the left — a 'white' moon-color is the core image. 洁 means 'clean/pure'. Together: 'pure white' = the moonlight's clarity.
