New iPad puts certain names to be texted in red and will not send message.
Certain contacts names turn red in thread and will not allow message to send.
iPad Air 2, iPadOS 15
Question marked as
Possible diagnosis: This can happen if those contacts have non-Apple devices that cannot be reached through the iMessage system AND you forgot to update the SMS forwarding on your iPhone to forward SMS message to/from your new iPad.
On your iPhone, check Settings > Messages > Text Message Forwarding to make sure the slider for your new iPad is enabled (green).