Removingtranslucent layer from banner

I have uploaded a logo for the banner on my website, but there is a grey layer over it which obscures the colours. How do I get rid of it? Also the height permitted for the background image in the banner is less than the 550 pixels it says the image can be. Why is this? The site can be seen at: https://e-access.co.uk/.

The header seems to be controlled by the id ‘masthead’ but I can find this in none of the css or JavaScript files. Where is it defined? Thanks for your help.