So Sketch has symbols. These are great if every instance of the symbol across your various artboards is exactly the same. But what if say you had a tab bar, and on different screens, you wanted to show a different tab highlighted/selected? I've been just having the tab bar as an individual element, not a symbol, and manually changing the tabs for each page to show the right tab selected. I feel like there is a better way to do this, as when the client changes their mind about a colour, I'd have to individually change every instance.
There's a plugin called state machine that solves your task.
Plugin Download: https://github.com/romashamin/statemachine-sketch
I don't know if you're familiar with sketch plugins. Sketch toolbox is a good way for managing plugins. http://sketchtoolbox.com/
Nested symbols can help you out here.
Make your tab bar with all tabs in the same state, e.g default.
Create a symbol from it - TabBar/Default
In the symbol page, insert an instance of the TabBar/Default symbol (not a copy) for each tab.
Make an active state for tab 1, position over one of these instances, and create a new symbol - TabBar/Tab1_active
Repeat for each tab.
If you later need to change the colour of the tab bar background, this now only needs to be done in TabBar/Default.
If possible, using styles or symbols for the Active tab can reduce effort for changes too.