Sadly, the art program I’m using doesn’t cover vignette effects. I imagine it’s older than most of the readers here… (1996)
It’d be pretty hard to sepia wash the top speech bubble there, since it bleeds into the gutters. (I do this sometimes when boxing it in cramps the panel, it’s a cheap and dirty trick.)
I have another “sepia flashback” coming later that I think works a little better, because (1) I actually drew the scenery, and washing that is more obvious, and (2) the flow of the narrative sets the reader up to expect a flashback.
While this is a common technique in storytelling, it’s not a technique I use a lot myself - mostly due to the format and layout of my comics being very short - and this is a technique that often demands space to set up and pay off.