What Slicer displays looks correct. This is a 3D rendering, therefore if you have a piece of red blob in front of a green blob, you will only see the red blob if you make Scalar Opacity Mapping function ramp up to 1.0. Even if you make the opacity plateau at a lower level (e.g., 0.3), whatever is in front will have larger influence on the color.
Another thing that you may not expect that when you render multiple volumes independently then the colors are combined using additive averaging (similar to a spinning colorwheel - brightness of colors are not added), and not using additive averaging (like in fluorescent imaging when you sum the intensities of multiple light beams).
As a result, you will not see bright yellow if you use multi-volume rendering (1. the yellow may be occluded, 2. if you crop the images to make the mixed yellow visible, that the yellow will be relatively dark).
If you want to see bright yellows then the best is to display the image as an RGBA stack. You can enable direct RGBA volume rendering as described here: Merge colored images and show them as 1 volume - #6 by lassoan.
If you only have RGB channels then you can generate an A (alpha = opacity) channel by averaging the RGB channels and generate an RGBA image by appending this A channel to the RGB image.