Almost any glossy coat will make the color darker, and almost any flat/matt clear coat will make the color lighter. Actually, it's true for the same shade of paint in glossy versus flat, or even for solid objects that are polished or left rough.
This is because of what makes surfaces glossy or matt. Glossy surfaces are smoother, at the microscopic level. This means they reflect light as glare, bouncing it relatively cleanly. Matt surfaces, OTOH, being rougher bounce the light less coherently, and instead scatter the rays.
It's like the difference between throwing tennis balls at a flat surface (the bounce will always be predictable, and if the balls come from the same direction, the bounces will all be the same) versus a pile of rocks (the balls bounce unpredictably, and the same throw will bounce in any number of directions, depending on exactly which piece of rock it hits). Light is the tennis balls, and matt surfaces are the pile of rocks.
So, when you Future/Kleer/glosscote/polish a model, you're getting all the light to bounce in the same dircetion. So, in that one direction, the model's much lighter, but we (correctly) interpret that as glare, not the color of the model itself. When you Dullcote/matt varnish/roughen the model, the light that used to be glare is scattered. Our eyes and brain percieve this as being a lighter shade, even though it's just a very diffused glare!
Bottom line: the model will get lighter again when you coat it with a clear matt finish. Sorry for the long post!