If you are trying to change the background color of your block and some of the block remains grey, the type of block you are working with may be the culprit. The grey background color in a block indicates that your block is a media block and is expecting you to add either an image or video clip to it. Any images or videos you add will cover the grey area of the block. The grey area won't change when you change the background color of your block since it is holding a place for media files.
If you don't want to add media to your block, you can change your block to a text block.