Photoshop colors

We have some graphics that were created (in Photoshop) that we've tried to
go into Photoshop and edit and the colors won't match up.  For example, we
have a nav bar that uses graphic buttons. When we try to change one of the
buttons, the color will no longer match the color of the other buttons on
the nav bar. Something about opening the file changes the color.

Has anyone else had this problem with Photoshop?  It's not really an option
for us to buy new software right now.  I'm sure there's just something we're
missing--a setting or something?

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