Black Bird Image Optimizer is a small app that uses 3 other other apps to do its work, partially or in full -- I'm not sure what contribution Black Bird Image Optimizer makes beyond providing the GUI. Google & you'll find all 3: jpegtran.exe, pngquant.exe, & TruePNG.exe. It may be interesting to note that one of the Google hits for TruePNG.exe talks about removing it from an app's Plug-in folder because it had been shown to be particularly ineffective.
In a quick test using a jpg image the After preview window obviously looked different, had Very different coloring than the Before window. Result unfortunately looked like that After preview. If you want to check just how much an image is changed by an app like this, a quick & easy method is to load the before & after images on 2 separate layers in an image editing app, then select difference for the layer blending mode. It'll do just what the name implies, showing you everything that's different about the images. That said, if you don't see such a huge color change, or if it doesn't matter for whatever you're using the new image for, Black Bird Image Optimizer might be OK.
Installation adds the program's folder, a folder under Users\ [UserName]\ AppData\ Local\, & in the registry, keys for uninstall and **I think** possibly activation. I wouldn't mention those other keys except for their odd location under Services, which are normally apps that run with Windows. They're under: [HKLM\SYSTEM\CurrentControlSet\services\ASP.NET_4.0.30319\Names]