Why are some buttons purple? Basically to see this, all you have to do is hold down the d key and hover the mouse cursor all over a node or modifier and many of the active elements will change to a purple color.

This sometimes happens to me when I've been using the grease pencil to make notes and then move over some part of the UI quickly before I remember to release the 'd' key.

So what is this purple, and how can it be un-purpled?
By pressing D over an input field you are telling Blender to create a driver for that value. A driver is a way of controlling one value (the driven value) by another value (the driver). For example you  could drive the scale of one object by the rotational value of another.

To remove the driver you can either right-click on the value and choose 'Delete Drivers' or you could:

Navigate to the 'Graph Editor' from any screen area:



Change the view from 'F-Curve' to 'Drivers':



Shift select all the driven channels in the list and press X to delete them:



If you want to view and delete drivers for multiple objects at the same time make sure to toggle off the cursor icon which limits the view to currently selected objects: