Most reflection nebulae are blue because the tiny particles of dust that reflect are very small.
Like particles of smoke, they preferentially scatter blue light rather than other colours, which is why most of the reflection nebulae here are bluish. Some, however strongly reflect the light of the star that illuminates them, such as that around Antares in UKS 30, while others reflect a brief flash of starlight from a supernova, such as the light echo of supernova 1987A, seen in AAT 66. In one case, the dust is reflecting the light of another nebula, that in Orion, and there is no blue light to reflect, so the reflection nebulosity in UKS 25 appears yellow. The Trifid Nebula in Sagittarius, M20 AAT 20. Nebulosity in Sagittarius, NGC 6589-90 AAT 34.