When the tank selected is in white text, it means you are in auto fuel burn mode -- this is the default mode when you launch an aircraft. In auto fuel burn, it automatically switches between tanks in a default order for that aircraft with that fuel load.
When the tank selected is in yellow text, you are in manual fuel burn. Only that tank in yellow text will remain selected and you burn fuel from that tank only. Important note: unlike auto fuel burn, when a selected tank runs dry in manual fuel burn, it will NOT automatically change to another tank and your engine will stop. You need to remember to switch tanks or to auto mode just before you run it dry.
Hitting Shift+F toggles between all the different fuel tanks in manual fuel burn mode (yellow) AND then back auto fuel burn (white).
Where this can come in handy is when you take a fuel hit and want to extend your range slightly by burning off that tank first (before it leaks dry). For example, lets say you are flying a P-51D and take a right wing fuel tank hit -- you can hit Shift+F to select your right wing (RW in yellow) to burn fuel from that tank before it runs out. Be sure to switch back to auto-fuel burn (white) just before it runs dry, or your engine will quit.
Hope this helps!