Thanks.

For Greenshot 1.2.x this is not an option, as the .NET Framework doesn't provide a way to do this (AFAIK).
See a blog entry about it here:
http://www.istartedsomething.com/20150217/microsoft-nets-jpeg-encoder-makes-crappy-jpegs/

We would need to add additional libraries, which do not work with .NET 2.x
So I will see what we can do with newer version,

Some people call it "color subsampling" other call it "chroma subsampling".
Please take a look at the wiki: https://en.wikipedia.org/wiki/Chroma_subsampling

I use Greenshot in conjunction with FastStone Photo Resizer because the latter has the option for "color subsampling". I use Greenshot to capture png files and then I convert them to jpg with color subsampling disabled (and set the quality slider to "100") to get the best quality. Filesizes are about 60% less while giving me nearly the exact same result.

It would be great if I only needed one application rather than two, hence my request.

Can you elaborate your feature request a bit?