As ID is a monochrome theme, most of the colour options are set within the customiser and modules.
There are some colour instances set within the stylesheet so if you want to change any colours in the stylesheet you can use this handy tool
Before making any edits to the theme, open the webpage linked to above and click on the ‘Choose file’ button, select the style.css file from the ID download package and then click on the ‘Load Data’ button.
You will then see a screen similar to this image. Select your new colours to replace the existing colours and then click the ‘Generate’ button.
You will be given a new stylesheet with your own colours to download. Click the ‘Open CSS File’ button and copy the entire contents of that page to your clipboard.
You can then navigate to Appearance>Editor and in the style.css file replace the contents with the contents of your clipboard and save. Your site will now be in your chosen colours.
Alternatively you can right click and save the file and then upload it to your site via FTP.