Hi @AlfMachine ,
I have completed ROS Basic course with Python. I had the same issue.
The laser mounted on the TurtleBot is a 360 degree laser. That is why the second picture of yours works.
Refer my own post here: Incorrect Robot Scan Values in ROS Basics Course - Wall Follower Robot / TurtleBot3
You figured it out yourself!
PS: Even I wanted to change the color of the walls. Even I could barely see where the robot was heading. But I still managed to complete the project.
Anyways, all the best with the rosject!
Regards,
Girish