A user in the Adafruit Forums posts:

Thank you Adafruit for your commitment to the maker community with your hardware and library creations/support. I was recently building a servo controller for my daughter to use with her LEGO Ferris Wheel project and while building the UI for the SSD1351 OLED, I came across the need to convert 24 bit color values to 16 bit packed values to use with the GFX library. I came across a locked thread here in the forums with some good examples of how to convert values, as well as many links to how others are doing it online—I just could not find an online tool though.

So, with that in mind I pulled out of maker mode and into web developer mode and created a simple color tool which gets you the 5:6:5 packed value. It also shows the bit values so that anyone interested in more than the hex value can see what the bits are doing. I want to add some more to it to educate users too—if they are interested. I need to refocus on paid work, so if anyone wants to contribute some instructions, I will evaluate it and add it to the page. I hope it saves someone a bit of time.

Very handy! You can use this free web tool at http://greekgeeks.net/#maker-tools_convertColor