Dark colors usually means not enough lighting, my son tells me. A wider aperture lens can help improve results under less than optimal lighting conditions. Sometimes you may want poor lighting for an effect, to bring the eye's attention to your subject. I would compare it to bokeh, having a blurred background to cause the eye to direct its' attention on the in-focus foreground subject.
A fully manually operated lens is best for shooting serious video. I have bought a couple of Rokinon "Cine" Series lenses for my son, and he loves them. I like them, too, because they are a good buy. They are rated using a "T" factor, instead of an "F" factor. The "T" rating stands for "Transmission", for how much light is actually reaching the sensor. As light passes through the various internal lens elements, it can lose some of its' intensity.
Unless you are intimately familiar with precisely how much light each of your video lenses lose due to passing through the internal lens elements, then when you switch from one f/1.8 lens to another lens set at f/1.8 you really do not know how much light is reaching the sensor. One lens may give you a slightly brighter, or dimmer, image than the other one. The "T" rating takes light loss into account. When you set a lens to T1.8, then you know the next lens you use that is set for T1.8 will allow the identical amount of light.
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