Since the file is owned by root:root, it will probably require changing permissions/ownership of that file so that the app can write to it
nemo ~ $ ls -l /sys/class/leds/led\:rgb_red/ total 0 -rw-r--r-- 1 system system 4096 Dec 31 00:03 blink_delay_off -rw-r--r-- 1 system system 4096 Dec 31 00:03 blink_delay_on -rw-r--r-- 1 system system 4096 Dec 31 00:03 brightness lrwxrwxrwx 1 root root 0 Jan 3 20:04 device -> ../../../pm8xxx-led -rw-r--r-- 1 root root 4096 Dec 31 00:03 max_brightness drwxr-xr-x 2 root root 0 Jan 3 20:04 power lrwxrwxrwx 1 root root 0 Jan 3 20:04 subsystem -> ../../../../../../../class/leds -rw-r--r-- 1 root root 4096 Jan 3 20:04 uevent