How to change the background color that appears at top when we pull down a website on mobile device?

This is simply the background colour of the body element. On your site it’s currently set as this:

body {
    background-color: #c1e7e7;
}

This appears to be being added by your theme, probably based on a customisation option somewhere. You’d be able to overwrite it by adding matching CSS in Appearance > Customize > Addition CSS, if you can’t find it.