α-Naphtholphthalein (C₂₈H₁₈O₄) is a

phthalein dye Phthalein dyes are a class of dyes mainly used as pH indicators, due to their ability to change colors depending on pH. They are formed by the reaction of phthalic anhydride with various phenols. They are a subclass of triarylmethane dyes. Common ...

used as a pH indicator with a visual transition from colorless/reddish to

greenish blue ''Icaricia saepiolus'', the greenish blue, is a butterfly of the family Lycaenidae. It is found from the northwestern United States to southern Saskatchewan and Alberta. The wingspan is 21–28 mm. Adults are on wing from June to August.< ...

at pH 7.3–8.7.

References

PH indicators 1-Naphthols Phthalides Triarylmethane dyes {{organic-compound-stub