Hey Krzysztof, thanks for this question!
The Color Selector and Gradient Editor has been updated since I created my FL Studio Beginner Book and FL Studio Beginner Course. (So this was the perfect time to update that info for everyone!)
(FL Studio 20.7 introduced the updated color selector!)
Please read my article/video I’ve created for you!
-> Using the Updated Color Selector in FL Studio
To answer your question quickly:
Simply just RIGHT-CLICK beside a sound on the Channel rack to highlight a sound. You can select and de-select sounds this way.
Then you can open the Gradient Editor by using the Keyboard Shortcut (SHIFT + ALT + C) .. or as you mentioned in the top-left arrow menu in the Channel rack (Step Sequencer) with “Color selected -> Gradient…”
If you’re still having issues with the Gradient not coloring properly, just reply back.. let me know if this helps okay?
Again, read/watch my color selector tutorial to help you out.
Thanks so much for being a member! 🙂