Hey Dutch,
Here goes...
1: Have a look at this Lumicon filter site. It explains everything about what EACH filter does, and their best application:
http://www.lumicon.com/astronomy-accessories.php?cid=1
2: In the visual world, we, as humans, are limited in how sensitive our eyes are to light. If the illumination, or brightness, falls below a certain level, we see things in "black and white", really in shades of grey. The colour of things is still there, just that our eyes cannot perceive them. Have you ever noticed this in a very dimmly lit room, like your bedroom with all the lights out? Another surprise our eyes have for us is that their most light sensitive region isn't our central vision, but it surrounds it. Remember hearing about "rods and cones" in our eyes? That is why "averted vision" is the trick to viewing deep sky objects - the central vision is packed solid with colour receptors, sacrificing light sensitivity,
. Light sensitivity is then a perifferal sense, which allows us to "see out of the corner of our eye", tiny variations in light is what this region is best adapted to.
Then there is the added variation in colour sensitivity, but this is of no consequence in astro-visual as DSO's are only seen in shades of grey.
3: Long exposure photography allows the colour of dim objects to be seen, again, not because they are not there, but we just can't see them.
However, if your scope is large enough, we can begin to see colour in only the brightest of DSO's, like M42, The Great Orion Nebula in Orion's belt. Through, say, a 17.5" scope, you can actually begin to see shades of pink tinge some of its filaments. It also helps to be in a dark site as light pollution also impeeds contrast. The Horsehead, in contrast, is a shadow imposed upon a very, very dim emission nebula. The nebula glows in red, but we can never see its colour as it is so dim, except in photos.
There is another thing about "colour" in objects, particluarly nebulae. There are two types of nebula: emission and reflection. Emission type glow because their gas is being 'excited' or ionised by the nebula's related star or cluster, and glow in the red end of the spectrum (the Horsehead nebula). Reflection, are just that, the gas is reflecting the associtated star light, this being blue in hue (the Pleiades). Then there are nebulae that have both emission and reflection components, so they glow both in the red and blue (like the Orion nebula). That is why there are various nebula filters listed in the Lumicon guide, different filters transmit different wavelengths according to the type of nebula you are trying to see or photograph.
Mental.
. No wonder Ron from Sirius says to me "Hi, Dutch, am I eating steak tonight?" Cheeky Bugger 
you have taken days of research away with that link and stopped the question regarding presenting images right in its tracks. I hope to catch up with you one day, Alex, because the first dozen rounds are on me. I deeply appreciate your comments and help.
Antares in particular (as it's difficult to split as it's so bright & large). It says the filter makes the two stars different colours making it easier to split the brighter star from it's counterpart. The brighter star turns red and the star turns blue. Interesting!
