QUESTION:
How accurate can you set the color filters to get a picture as close as
possible to what our eyes would see.

ANSWER from Heidi Hammel on April 24, 1996:
We can measure the colors of Neptune very accurately using an 
astronomical instrument called a spectrograph, which often uses a prism 
to spread the light from the planet into a rainbow, and then records the 
colors of the planet rainbow with extremely high precision.  With the 
information from that observation (which can be done with a regular 
telescope on the ground; you don't need Hubble for it), we can figure 
out how to "transform" the images taken with the Hubble's filters into a 
"true" color picture.