Use system fill colors for items situated on top of an existing background color. System fill colors incorporate transparency to allow the background color to show through.
Use this color to fill large shapes, such as input fields, search bars, or buttons.
と説明されているとおりだった。
ライトでの cgColor の出力はこんな感じ:
<CGColor 0x60000260e1c0> [<CGColorSpace 0x60000260cf00> (kCGColorSpaceICCBased; kCGColorSpaceModelRGB; sRGB IEC61966-2.1; extended range)] ( 0.462745 0.462745 0.501961 0.12 )
