The claim that atmospheric scattering causes yellow and pinkish skies (such as during sunrise and sunset due to Rayleigh scattering) is a matter of everyday observation and textbook physics; no citation is needed.
The claim describes a basic, universally recognized optical phenomenon (Rayleigh scattering of sunlight through the atmosphere producing red, yellow, and pink skies at dawn and dusk). Under STEP 0, trivially true statements of direct everyday observation are categorized as COMMON_KNOWLEDGE, requiring no papers and leaving the index lists empty.