Without exact system information, render settings and so on nobody can tell you much. That aside, there are some fundamental failures in your plan. First, web colors mean nothing in AE or in most graphics programs for that matter due to the little subject of color management and possible quantization errors. Whatever colors you dial in, do so using the normal RGB sliders. Second, blending modes do not create transparency, even if for mathematical reasons the blending formula used may appear to eradicate certain colors. Third, you need to consider Gamma values. Video is not blending linear and both your computer screen as well as the video content itself may have a built-in Gamma. As a result, your blending operation could already produce wrong results like a slight haze, the encoding of the video file could introduce Gamma skew and then finally in the last step, whatever tool you use to display may apply dynamic corrections as well or in reverse, none at all. So it basically comes down to checking those things and properly calibrating/ adjusting your monitor and your system color settings and make use of color management in AE, if relevant. In the end, though, you will always see minor color deviations. It's inherent in how compressed formats like H.264 work. You can only try to minimize them as much as you can.
Thanks of the heads up Mylenium. I will take what you have said on board and see how close I can get using your advice.