`1b` is **Escape** represent by 2 characters `^[` which can be invoked by <kbd>Ctrl</kbd>+<kbd>V</kbd> follow by <kbd>Esc</kbd>:

 xb@dnxb:~/Downloads/grep$ ascii 1b
 ASCII 1/11 is decimal 027, hex 1b, octal 033, bits 00011011: called ^[, ESC
 Official name: Escape
 
 xb@dnxb:~/Downloads/grep$

Do this (Use <kbd>Ctrl</kbd>+<kbd>V</kbd> follow by <kbd>Esc</kbd> to create `^[`, then continuously type `\[41m`):

 xb@dnxb:~/Downloads/grep$ hexdump -C /tmp/2
 00000000 1b 5b 33 37 6d 1b 5b 34 31 6d 30 2e 30 30 25 0a |.[37m.[41m0.00%.|
 00000010
 xb@dnxb:~/Downloads/grep$ \grep '^[\[41m' /tmp/2
 0.00%
 xb@dnxb:~/Downloads/grep$ \grep '^[\[41m' /tmp/2 | hexdump -C
 00000000 1b 5b 33 37 6d 1b 5b 34 31 6d 30 2e 30 30 25 0a |.[37m.[41m0.00%.|
 00000010
 xb@dnxb:~/Downloads/grep$ 

Ensure you escape grep to avoid its alias with `--color` affect.