Filters filter out light. White light is a combination of all wave lengths of visible light -- remove some of that light by using a filter and you have less light hitting the film. Some objects reflect a limited spectrum of light (if that is the correct way to say it). Thus an object that reflects primarily red light, would be darker when photographed thru a blue filter -- it would be "underexposed" relative to the rest of the scene. But the rest of the scene also is reflecting light that includes red (such as white objects, orange, brown, etc.) giving the film less exposure over-all.
I suppose even that red object is reflecting some blue light, tho only a small amount -- thus we have different shades and intensities of "red".
In theory, a scene of nothing but blue objects photographed thru a blue filter, would not need any filter factor applied. But scenes are typically full spectrum.
You are correct, film sensitivity is not changed by filters -- the filters reduce the amount of light reaching the film. Film is more sensitive to blue light -- use a red filter and you have eliminated a major portion of the light the film is most sensitive to -- thus one of the reasons that the filter factor for red filters tend to be highter than blue filters (along with the large amount of blue light that makes up a daytime landscape.)
I use a yellow filter to photograph the Fall colors in the redwoods (only yellow really -- not the East coast variety of colors). The yellow filter reduces the exposure of the blues (and also greens, purples and whites). Rather than have most of the scene underexposed and the yellows unaffected, I increase the exposure to bring the blues, greens and purples back to their proper exposure and the yellows are "over-exposed" and create wonderful hightlights.
