黄不溜秋 follows a productive northern colloquial pattern: color + 不溜秋 / 不拉几 = "an ugly version of that color". Compare 黑不溜秋 (dirty black), 灰不溜秋 (dingy gray). Always negative, never use it for a pleasant yellow.
register
Strongly colloquial; mostly heard in northern China. Carries a tone of distaste.
