The “Luminance” channel in the histogram in Shotcut is actually the “Y” channel from the rendered YUV image. The Red, Green and Blue channels are converted from the rendered YUV image. I had considered making a YUV histogram also. But I don’t think most people would know what it is or how to use it. Most (all?) NLE color grading workflows operate in the RGB space.
Conversion from YUV to RGB is not mathematically lossless because not all YUV values can be mapped to the RGB clolor space. But video images that come from a camera originated in RGB because camera sensors are RGB. So unless your source material originated from a non-RGB source and used YUV values that don’t map to the RGB colorspace, you shouldn’t have any pixel values that get grossly modified. Any loss in the conversion should be limited to rounding errors of a single bit.
For Shotcut, we aspire to limit the pixel errors to one bit. There is a relevant thread here:
We also aspire to keep the user interface clean and intuitive. So I think it is unlikely that we will offer multiple versions of the same filter just because they operate in different colorspaces. So we would need a good reason to switch to using a different filter on the backend. I’m not sure if a pixel error of “1” is enough reason to change the filter. Maybe.
While it is mathematically lossy, it is not visually lossy. So I guess you would need to decide where your priorities lie. For Shotcut, this has not been a compelling issue, so we have been fine with colorspace conversions.