When you have to deal with multiple NetSuite environments (and, as usual, are trying to move fast and satisfy the needs and demands of multiple stakeholders), it can certainly get quite confusing quickly. Am I in sandbox or production?
Sure it says “Sandbox” quite clearly next to the NetSuite logo but that can easily be overlooked and now you’ve either just spent several wasted minutes (or, heaven forfend, hours!) inputting data into the sandbox or worse yet just deleted several (or several thousand!!) critical records in production. Ouch either way!
Simple solution? Change the color scheme as soon as you refresh your sandbox! That makes it easy to know at a glance which instance you’re in, since the color permeates the entire UI.
To change the appearance, head over to your Home > Set Preferences > Appearance tab and set the color scheme.

Here you can see I have production set to a cooling Moss Green

…and Sandbox I have set to black.

Easy, elegant, and (hopefully!) foolproof.