Is it possible to pipe(afterthought apply function) with options?

Because currying is not performed automatically. But you can do the following:

Image@hexToRGB["#99c361"] // DominantColors[#, 1, "HexRGBColor"] & 

You can define an operator form for DominantColors by modifying its definition:

Unprotect[DominantColors]
DominantColors[s_String] := DominantColors[#, 1, s] &; 
Protect[DominantColors];

A better/safer alternative is to define your own function with its operator form that works like the function DominantColors:

ClearAll[dominantColors]
dominantColors[a__, o : OptionsPattern[DominantColors]] := DominantColors[a, o]
dominantColors[s_String] := dominantColors[#, 1, s] &

"#99c361" // hexToRGB // Image // dominantColors["HexRGBColor"] // First

"#99c361"

First @ dominantColors["HexRGBColor"] @ Image @ hexToRGB @ "#99c361"

 "#99c361"