Hi,
Setting a node colorspace is valid only for the scope of the node - the output of the node goes back to timeline colorspace. So an HSL node (with luma set to 0) will change to timeline (RGB), resulting in a luma=0 (i.e. black) image output. You can observe the same (correct, black-and-white) behavior with Resolve 16 if you were to toggle this node's space to timeline and back to HSL.
I think I understand this part. However, in the video I attached, the 'Chroma' node is almost black but the results are fine. Davinci Resolve 17 hasn't the same behaviour.
The solution is to use two colorspace transform nodes (timeline to LAB and back) to keep the mix in LAB - this lets your setup work as intended.
1) Where should I use these two color space transform nodes? I've tried several combinations unsuccessfully.
2) Which should be the settings of the first one? I used these settings: Input Color Space: Use Timeline; Input Gamma: Use Timeline; Output Color Space: Lab (CIE); Output Gamma: Use timeline. Are these the correct settings? Should I left Gamma untouched? Should I change anything on Tone Mapping, Gamut Mapping or Advanced settings?
3) Which should be the settings of the second one? I used these settings: Input Color Space: Lab (CIE); Input Gamma: Use Timeline; Output Color Space: Use Timeline; Output Gamma: Use timeline. Are these the correct settings? Should I left Gamma untouched? Should I change anything on Tone Mapping, Gamut Mapping or Advanced settings?
Best,
Sebastià