My comic has a HUGE focus on flashbacks, so to separate the large-scale flashbacks from the current time (and as to not get readers lost), only the comic is fully colored. Most of the memories or special sequences will be in BW, a bit of grayscale or colored areas where needed, or a different artstyle entirely.
For smaller flashbacks though, I tend to just make them painterly and with little to no lineart, as if it's a background. Here's a page that demonstrates both, though the BW part is a test and is just that one panel:
Feathering is a little too dreamlike for most flashbacks, and keeping the style ignores that a lot of people will take breaks between pages and forget entirely what's happening in the next one. If there's a clear indicator that it's not "now", it's much safer.