- @Living Breathlessly
Let's start with shadows and darkness. A shadow is caused when something is blocking the light but it's not a hundred percent darkness because you can (usually) still see whatever is behind it, so while it's absence of light, there's still light in it. Like how different levels of intensity of light will change the light's brightness, the different amount and intensity (and source too probably) of light will affect what darkness. In this case, the less light there is, the more intense the darkness will be, so if there simply is no light, then it would be complete darkness.
Manipulating darkness: I assume it would be manipulating what and how much light you take out of a place. Imagine a chalkboard that's covered in white chalk. The chalkboard is darkness and the white chalk is light. To see the blackboard, you have to erase the white chalk covering it. If you only lightly erase the white chalk, you'll be able to see some of the chalk yet also see the board behind it. If you completely erase the chalk, then you would only be able to see the black of the board.
Moving onto light. Light can be a weapon and for some reason, it doesn't really seem like there are that many people who realize it when writing stories and roleplaying and the likes. Most people seem to focus on the illusion aspect of it, and tend to stick to great, magical feats involving illusions in their stories. In a combat scene for example, the light manipulator would send out copies of themselves, make their opponent(s)'s worst fear come to life, hide an entire army with their powers, etc. Instead, why don't people just use it to, let's say, make it seem like everything is moved 5 cm to the left to their opponent? Or 2 degrees up? This is subtle enough that it will take longer for people to notice, and it will be much more difficult to get rid of. Even if they get used to their change in perception, all you would have to do it take away the illusion and they'll be dazed and confused in no time.
Now, let's move onto another aspect of light. Ever heard of lasers? A form of light amplification make them work (I don't know the specifics btw). Telescopes, using lenses and mirrors to focus and reflect light so they can see things from far away. Heck, glasses use lenses to focus or disperse light to help shirt-sighted (convex) and far-sighted (concave) people to see. A very plausible technique that light manipulators could use is using lenses and mirrors to change their power, whether it is to focus is (convex lenses), spread it (concave), or to reflect and magnify it.
Light does have physically damaging properties (also, ever heard how bad it is to look into the sun or any source of bright light?) and this aspect of light seems to be woefully underused (I read this Harry Potter fanfiction with a dark Hermione Granger who used an overpowered lumos to cause someone to be blind for over 3 weeks which I loved). Of course, an obvious use of light is to cause someone to be (permanently or temporarily) blind (an example is in that Doctor Light episode in the Flash). And as lasers have shown, if light is intense enough (I'm working with a very simplified explanation of lasers here), it can cut through things, diamonds for example (in a truly wonderful trilogy, Shadow and Bone by Leigh Bardugo, the protagonist is a light manipulator and has this ability called the Cut which, well, cuts. While it was implied that the Cut wasn't from her light manipulation abilities since her counterpart was someone who controls darkness and was essentially the only other person able to use the Cut. However, considering what I just said about darkness, it is perfectly possible that a very intense slash of pure light would have been able to chop a mountaintop off. Another is to spy or see things from places that you wouldn't be able to see from where you're standing. Telescopes help us see far away but only if certain light can reach it and light always travel in a straight line. Now, imagine a light manipulator who can get light to travel off-course and get it to some sort of device (or turn it into an illusion screen thing) that can show what is happening wherever the light came from.