How can I change this color?

How can I change this color?
Where can I modify it?

You can override with this custom CSS rule:

.horizontal-nav {
    background: #465994 !important;
}

change to your desired color and put this code to one of these 2 files on your site (create if it doesn't exist):

root/templates/ja_colab/css/custom.css
root/templates/ja_colab/local/css/custom.css