Edit: I found the place to edit the messages colors. It requires an additional software to edit .db files. The file is located here:
/data/data/com.textra/databases/messaging.db
then, open the contact_settings table. You'll find two entries similar to the themeMaterial String I said in the last post. To edit those values, use the same algorithm than before.

WARNING: changing those values or the themeMaterial values will makethe settings part unstable, you won't be able to change those colors from the app anymore

Hey there, I was looking to do the same, and I found out how to do this for the main and accent color, but not for the messages color for now. Here's how:
- got to /data/data/com.textra/shared_prefs/ (needs root)
- open the com.textra_preferences.xml
- if you know XML, then just jump to the themeMaterial string
- else find the line with <string name="themeMaterial">
- the first number is the main color and the second os the accent, to create such a number you just have to do this small operation:
R*65536+G*256+B-16777216

Hope this helps!