'White' text is grey in a Colormanaged environment?

I guess I have the same questions of a loader node and also a generator such as Text+ or a background node's color. What color space / gamma are these outputting?

Those just generate RGB values. They are not doing anything in a special color space or whatever. It's just values. You tell the software how to interpret them. But.. (see below)

I understood that Fusion operates in a linear gamma, but I don't know where that conversion occurs, nor do I know if it gets inverted by the MediaOut node or whether we have to invert it ourselves (either on the Fusion page or in the Color page).

In RCM2 resolve expects the fusion comp to be in linear and in the timeline gamut. Therefore you could say the generators produces values in this space. But you can easily manipulate it by adding a CST inside fusion just before the MediaOut.
Does that make sense?

About the conversions. We're talking about several things here. Conversions happening inside of fusion and conversions before/after fusion. The one before is the IDT you set the footage to in the Media Tab. It goes camera color space to timeline color space. So now everything is in timeline color space.

In Fusion there is actually just timeline gamma to linear (start) and linear to timeline gamma (end). It does not care about the original camera color space or the display output color space.

And then there is the ODT at the end of the entire chain.

This is a great explanation and has moved me forward in my understanding of the implicit color space / gamma conversions within RCM/Fusion. What about in non-RCM projects? Does MediaIn still convert to a linear gamma and in that case, what is its implicit color space - is it that of the media or is it still assumed to be the timeline color space? Thanks so much for your help, Sven, as well as to others on this thread.

What about in non-RCM projects? Does MediaIn still convert to a linear gamma and in that case, what is its implicit color space - is it that of the media or is it still assumed to be the timeline color space?

In unmanaged Fusion does no conversion on its own. You have to tell it how to interpret the footage in the Inspector (Remove Curve) or with a separate CineonLog Node.
Definitely do not use the Curve Type "Auto" on this. Because it does not read the correct color space tag from the metadata. Use "Log" instead and tag it accordingly.

It looks like it does a simple log to lin conversion, but in Auto-Mode it thinks the footage is Rec.709. Sadly it does not take into account the timeline color space tag, that you can set in the project settings. So if you're after correct color transforms, do it manually!

Thanks Sven. Looks like with Text and background colors (in a non-RCM project) I can just convert to a linear gamma using a CST setting input color and gamma to DWG and then output color to DWG and output gamma to linear and then these can be mixed with colors from the video media (also transformed to DWG/linear) and then at the end of the pipeline just before MediaOut, a transform back to DWG/intermediate.

I do find with the loader node in an RCM environment that I need to play with whether or not to tone map and if so, how high to set my max output nits... Still a bit of guess work. And then, if 'User DRT for SDR to HDR' is disabled in the project settings... well, many things change. I'll keep at it for a while. It's almost 5 o'clock pm here - time for a drink, lol.

Add - I took what I learned on my PC and then tried the same on my MacBook and found that graphics elements such as text and background could not be converted into linear in a non-RCM clip. Still investigating what the difference is between my PC setup and my MacBook - both are running 18.1.1 - could be a project difference, I suppose. Something to investigate tomorrow. Yippee.