When change view into rendered, the object turning grey. I have followed this tutorial recently:https://www.youtube.com/watch?v=cLUAp4lSxLI

But I had the problem at 30:12. When he changed viewport into rendered, the object is clear: 

However, I did the same as he did, but my object looks grey:


Had I skipped something important?? What's that???
In one of the previous videos he had assigned an HDR image as a light source.

In the world panel click the button right of the dark grey colored box


Then choose environment texture


Then load Your HDR image:



You can find these HDR images for free at HDRIhaven
An HDR image is basically a spheric image with awesome light information