Hack with confidence.
Page set-background-color.php is vulnerable to at least the following:
HTML Injection: It is possible to inject your own HTML into this page because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject HTML, CSS, and/or Javascripts in order to alter the client-side code of this page.
Cascading Style Sheet Injection: CSS styles can be used to interpret and execute Javascript. If styles can be injected, it is possible to inject a style with embedded Javascript which will be executed when loaded into the users browser.
Reflected Cross-Site Scripting: This page is vulnerable to reflected cross-site scripting because the input is not encoded prior to be used as output. Determine which input field contributes output here and inject scripts. Try to redirect the user to the capture-data.php page which records cookies and other parameters. Visit the captured-data.php page to view captured data.