The terms additive and subtractive refer to filter changes and are not applicable to the colors themselves in real terms. They depend on if a light source or filter is used to effect change. Its more instructive to think in terms of opposites.
Red/cyan
Green/magenta
Blue/Yellow
Think of an equilateral triangle.
Now cyan is opposite red and therefore can be used to
correct red.
Green is opposite magenta and so can be used to correct magenta.
And so forth. The triangle is less confusing than the wheel and makes the opposite of each
color immediately apparent.
Except for cameras with matrix metering capabilities most light meters measure for a mid grey and try to reproduce that tone no matter what you point them at. A blackcat in a coal shed will come out mid grey and so will a white wall. Assuming processing is standard of course.
So if you point your camera at a white wall the negative will be underexposed. In the print it will be, not white, but grey. If you point it at the cat the negative will be over exposed. Not a black cat anymore, but grey. Just like the shiny blade and the micarta grip on the knife you are trying to shoot.
Even though its the same light falling on the grip and the blade, your meter reading will be radically different depending on what you point it at. And neither reading will be correct. So you have to take a reading from something that reflects the mid grey that your meter is trying to reproduce. Or you have to compensate by overexposing when your meter is pointing at highlights to make them whiter and underexpose when its pointing at shadows to darken them down a bit.
If you process your own film or know a friendly processor you can get even more control by tying your development time into the equasion. When you process film the highlights move a lot faster than the shadows.
So you can base your exposure on the shadow density, and then with testing develop for the highlight density. This allows you to pull in contrast ranges that would normally yield harsh negatives and prints or stretch contrast ranges that are lackluster and flat.
Film does not see contrast the way your eye does. Your eye has no problem pulling in the fiber of the black linen micarta or the mirror polish of the blade with Your name delicately etched thereon.
But the film only resolves detail over 5 stops for color and 7 for b/w. The film thinks your knife is contrasty and harsh. So if the knife looks really good in a certain light take steps to reduce the contrast such as using a reflector or a fill in light source for the shadows. Or by cutting down on your main light if you are shooting inside. In that way the values for the shadows are pulled up close enough to the highlights that the film can handle it.
In other words make the knife look a bit less contrasty than you would like and it will come out more contrasty than it looks. :-)
One more thing. Think about whats around you when you shoot. If there is a pink wall, over there, the film will see a tint of pink in the blade even though the wall is not even reflected in it. The sky makes blades blue. Trees make them green. The best is if you can shoot somewhere that is monochrome, either white, grey or black.
When I talked about film resolving detail over 5 to 7 stops I was referring to contrast range, not exposure latitude. In other words if a scene is more contrasty than the film can handle, you will have to sacrifice detail in the hightlights or shadows or both. Unless you make the adjustments I talked about regarding push/pull and exposure compensations etc.
A grey card is a surface that reflects the exact tone your meter is trying to reproduce. To make an exposure compensation such as I recomend, BASED on a grey card exposure would really mess things up.. That is why the compensations I suggested were designed for meter readings at the end of the tonal range not in the middle. Not every guy has a grey card and must base his meter reading on a tone that is less than ideal.
My comments were designed to allow the guy to read any tone in the environment (with practice) and translate that tone into an accurate reading based on prior knowledge regarding the effect that tone will have on the meter and the appropriate compensation required. Whew!
Try this!
Set up three cards, a white, a grey and a black.
Make sure the light level falling on each card is the same. (Do an incident reading or do everything outdoors, whichever.) Now! Take a reflected reading from each card. (Preferably use a narrow angle meter to avoid noise.)
Say for instance that the reading from the grey card is F8 @ 125
The reading from the white card will be around F8 @ 500
And the reading from the black card will be around F8 @ 30
Depending on the contrast of the lighting and the reflectivity of the individual pieces of card. So all numbers here are approx not final. OK! Now We all know that the reading from the grey card is the correct one. But if there is no grey in the scene and we do not have a grey card we must base the exposure on another tone. If we know how many stops from mid gray that tone is, we can compensate accurately for the meters stupidity and dispense with the grey card entirely. Grey cards mimic the average scene. But a reflected reading from a knife is not average because of the contrasts involved. Say the reading is BASED on the blade.
The meter will give us F8 @ 500 if its in the same lighting situation as the three cards and the blade has the same reflectivity as the white card. But the correct exposure is F8 @ 125 Now if the reading is based on the micarta the situation is similar. The meter reading says F8 @ 30 and again we know that the correct reading is F8 @ 125
Let me illustrate it another way. Say you are out by the beach with your lady, and its a glorious day and you want to take a picture. But you got a problem, the sky is bright, the sea is bright, the sand is bright, and you have no grey card to take a reading. Furthermore your sweetheart is shy and you know she will never let you bracket. But you have one piece of info that allows you to take the picture. You know that caucasian skin is one stop brighter than mid grey. So you casually take a reading from the palm of your hand, making sure that the light is falling on your hand the same way it does on your beloved. Now based on that reading and because you know that "white skin" is 1 stop brighter than mid grey you open up the aperture one stop and you have the exposure nailed. No need to bracket.
Now if your name is Brett Weston you can simply throw the meter away, squint your eyes, call it on the nose first time and every time. The rest of us need to use a meter. He made Ansel Adams look like a schoolboy with a brownie.! Mind you he burned his negatives to hid the evidence. I don't mean "burned" as in printing. I mean he lit them up in a fire on his 70th and odd birthday. What a guy! :-)
The principals I elucidate refer to B/W specifically but are also applicable to color, to a lesser degree.
