When you open the custom color picker on a control in design, if you do not close it, it remains active on the last color edited control when you switch back to play mode. This is good when you are trying to fine tune a control's color - lets you adjust colors and see real time impact in play mode.
I was trying to generate an "invisible" control, this was perfect as it lets me cycle through until I have a match close enough that the control is barely visible
Images
Color picker active in play fine tuning 'invisible' control