In renders, all objects reflect the *World color*.   
White objects are affected most, they reflect it almost perfectly. Like in reality.

**Solution 1 - Emission node**  
Use the Emission node for your object's color.   
That way, the object will not receive any shadows or color tint.  
It will just be a silhouette emiting white light.  
Might work well for 2D animation.

*Add > Shader > Emission*

[![enter image description here][1]][1]



*Comparing different materials.  
Even colored materials get a slight tint.* 

[![enter image description here][2]][2]


**Solution 2 - Using the compositor**

[Available here on Blender Stack Exchange][3] by the legendary [cegaton][4] himself.

Cegaton's answer offers a different approach using the compositor.  
Rendered objects keep grey shading, but the background color can be changed at will.


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Try what works best for you :).


  [1]: https://i.sstatic.net/WOdzh.png
  [2]: https://i.sstatic.net/s98gO.png
  [3]: https://blender.stackexchange.com/a/108870/78972
  [4]: https://blender.stackexchange.com/users/1853/cegaton