Hi Pim,
Mixing Color in your setup is like layering "Gels". With this, the first one works like a filter. For example, if you have a red filter and a cyan (second) filter, they will block light simply based on frequencies.
However, what is a pure green and a Pure red? If you use an sRGB colorspace, you get something, but not a full saturated color like green or red. Yes, I know, everyone will say, but I can see it on my screen. Well, typically not. Which is a longer story. But a pure red in sRGB is in sRGB 8/bit values 255; 0; 0, but that is the smallest colorspace possible and never should be used in professional pipelines. This means if I take these 8/bit values, the red in ACES 2065-1 would be 112; 23; 4. This means not even a fraction of that saturation that the sRGB or even P3 screen shows. So, a red would not block green fully as it has green in it. So, mixing colors is a little bit more complex. (So, using ACES 2065-1 all the way, well, yes and no. Typically, a standard screen can't handle the colors, some Applications have problems (hence the ACEScg limitations), and the need for Gamut Compression and, later on, Ton-mapping changes the colors to fit into a smaller space, that many people answer with creative (to say it kindly) workarounds. In other words, it is a theme that takes a longer time to dive fully into it. I can share some book tips, or (my favorite) as I'm an Alumni of the Color Academy for professional Colorists, Los Angeles, to invest a year in their program.
But well, if the light goes through those, and both have perhaps a white background, the light would be a colorful ( or less so) shadow. In any way, this shadow moves back through both filters and is affected again. Like every color filter is just a partial ND filter. Note: colors are not real; we produce them inside of our brain. Light has only frequencies, which different cones can receive more or less, and then Color is created later. In terms of filter, some frequencies get more or less blocked, others don't. Why do I explain all of that? Because feeding Color into the Opacity means to create a filter. While red will create a Cyan-looking result. Think of the red channel, if red it the color resulting from RGB as a channel that shows white. Hence Blue and Green are black, resulting in Cyan as transparency. If inverted only red is a black channel, and the result looks like red gel filter.
In your scene, there is the top layer set to be a filter, and using the colors to drive the Opacity means the red is more transparent. I would suggest here a neutral gray, perhaps 70%. If the colors have to drive this, use an Invert to get the proper colors of the '70s.
Tip: Enabling the View - Options> HQ Noise might help to get improved previews (not related to the Color question)
In short, that was a deep dive for that short question, but I wanted to share more insights into what is going on - instead of just dumping a file and telling you to do this. Which I find always for those themes not adequate. Read it and let it sit for a while; it is not a small subject.
Here is your file back
CV4_2025_drs_25_RScf_01.c4d
Screenshot 2025-06-27 at 2.33.54 PM.jpg
(Blur would be a new thread.)
Enjoy
