NGC 6781 is a 1.8 arc minute mag 11.1 planetary. The disk is approximately 2 ly in diameter.
I'm seeing a number of images on the internet
with an intense blue center so I decided to calibrate the color. I could
find a G2v star in the 2003 color data so resorted to using SBIG's published transmission data for the filters and the QE of the ST-10XME chip.
Filter Mean transmission frequency QE at mean transmission frequency Color ratio
450 @ 80%
Green 525 @ 90% 77 1.00
Red 650 @ 90% 85 0.91
The result is the color above.