As far as I know, LORRI is panchromatic (black-and-white) and Ralph has red, blue and two infrared filters but no green filter. As such, I wonder whether New Horizons is capable of making natural color images.
Not really, but by being really careful one can get a reasonably close approximation. There are a number of cameras on board, with different wavelengths. The ones that cover visible or near visible wavelenths are (Source):
- RALPH "Blue" channel 400-550 nm.
- RALPH "RED" channel- 540-700 nm.
- RALPH "Pan" (Clear filter)- 450-1000 nm
- LORRI- 350-850 nm
Okay, so what qualifies as "Natural Color"? I'm going to say that is best done close to the human eye response. Wikipedia has a table with these response values:
- S 400–500 nm
- M 450–630 nm
- L 500–700 nm
Note that there is considerable overlap between the S and M. These are basically blue/green. Given all of this, RALPH can produce a good 2 color approximation in human wavelength, but not really all 3 colors. Some good approximations can be made, however, if one really takes the time to get it close.
Why not natural color? The colors that were selected were chosen to give the most interesting scientific results. Near infrared gives some very interesting information that can't be picked out in visible. Also, the blue overlaps so much with the green that there is little new information by including both bands.
Still, even most color imagers don't quite get the color right. I won't post the image (Not sure about the copyright), but this site contains a the spectral response of the Canon 40d camera, a moderately high end color digital camera. It sees far beyond human vision cutoff (700nm), although in a camera this is typically filtered out at the sensor level with an IR filter. Still, few people would doubt that this camera produces near true color images.
Bottom line, New Horizons doesn't have a "True Color" image, but it can make some pretty close approximations.