Follow the below steps to remove Theme: ColorMag by ThemeGrill. Powered by WordPress. from your blog footer. 

Login to your WordPress Dashboard.
Go to Appearance => Themes => EditorSelect inc/functions.php from the right side bar.
Search for colormag_footer_copyrightand delete the following code.

Quote
$wp_link = '<a href="http://wordpress.org" target="_blank" title="' . esc_attr__( 'WordPress', 'colormag' ) . '"><span>' . __( 'WordPress', 'colormag' ) . '</span></a>';

Quote
   $tg_link =  '<a href="http://themegrill.com/themes/colormag" target="_blank" title="'.esc_attr__( 'ThemeGrill', 'colormag' ).'" rel="designer"><span>'.__( 'ThemeGrill', 'colormag') .'</span></a>';

From the below code, delete the code highlighted in bold:

Quote
$default_footer_value = sprintf( __( 'Copyright &copy; %1$s %2$s. All rights reserved.', 'colormag' ), date( 'Y' ), $site_link ).'<br>'.sprintf( __( 'Theme: %1$s by %2$s.', 'colormag' ), 'ColorMag', $tg_link ).' '.sprintf( __( 'Powered by %s.', 'colormag' ), $wp_link );

After deleting the above code which is highlighted in bold, ensure that you have the following code:

Quote
$default_footer_value = sprintf( __( 'Copyright &copy; %1$s %2$s. All rights reserved.', 'colormag' ), date( 'Y' ), $site_link );