Hi, I tried this function on the original theme, but could not get it to work in the child theme.

Both td_wp_booster_functions.php and td_module_single_base.php is present in the child theme.

What code do I have to include to have it recognized from the child theme?

Thank you, but that did not work, so for other users I did it like this:
.td-post-template-default .td-post-sub-title,
.single_template_1 .td-post-sub-title,
.single_template_2 .td-post-sub-title,
.single_template_4 .td-post-sub-title,
.single_template_5 .td-post-sub-title,
.single_template_6 .td-post-sub-title,
.single_template_9 .td-post-sub-title,
.single_template_10 .td-post-sub-title {
color:#000000;
}