I keep trying to change my foreground and background colors in SecureCRT® 3.4 (or later) for just one session, but when I do, the colors for all of my sessions change. This didn't happen in 2.x versions.

What is most likely happening is that when you attempt to change the session colors, in actual fact, you are changing the values for foreground and background color for one of the globally defined color schemes.

To have each session use a different combination of foreground and background colors, you will need to create a color scheme for each color combination you wish to use.

For example, if "Session1" is using a custom color scheme named "blue-black" and you want another session to have a red foreground and a black background, you would create a new color scheme (e.g., named "red-black") and have Session2 use that color scheme.