To edit the color of the bar at the top of the sign-in page and sign-out page using custom sign in pages, search for the below code in LoginPage.thtml and Logout.thtml.
<td bgcolor="#0000FF"><img border="0" src="welcome.cgi?p=logo&signinId=<%signinId%>" alt="Logo"></td>
<TD bgcolor="#0000FF" align="right"> </TD>
You need to change the bgcolor to the Hexa color code you would like.
Please find the link which gives the details on the Hexa colour.codes https://www.color-hex.com/