If the clothing you purchased are currently in used by the Chinese Army, and you intend to wear it to deceive, then it is illegal.
Under HK Laws. Chap 228 簡易程序治罪條例 第21條 任何人穿並無資格穿的制服屬不合法
(1) 任何人穿着其並無資格穿着的制服,或穿着非常類似該種制服的衣服而刻意欺騙他人,可處罰款$1000或監6個月
This is because the army also have garrison in Hong Kong(中國人民解放軍駐香港部隊)