"instant boot" to gray scale, then full use

28Apr2015

iMac: 21.5, late 2013, OSX 10.9.5

I occasionally disconnect my iMac and store it or disconnect it when lightning is threatening (we live in Florida). Lately, the last couple of months, when setting up the mac again out of storage or disuse, I hit the power-on button and the screen immediately displays in gray scale everything: the wallpaper, all the folder/file icons, and a progress bar below. In less than 30seconds, the color snaps back, and all files, and programs that were launched before I shut down are now “instantly” available. Please explain what is going on.

Thanks.