On the Windows 10 Lock Screen, you can have either a picture or a slide show. If you would like to have a solid background, there is no option for this.

Here is what you need to do:

Click on the Start Menu and then type

lock screen

Choose "Lock screen settings"

Make sure that under "Background" you have chosen "Picture"

Under the pictures, choose "Browse"

Browse to C: > Windows > Web > Screen

Choose the blue solid picture (this is called "img105.jpg" on my machine).

Click on "Choose Picture"

That's it. You now have a solid background on your lock screen.