For testing the value of the template I just created this quivalent:
- service: mqtt.publish
data_template:
topic: "DIODER/CMD/RGB"
payload: >-
[{{ states('input_number.led_strip_r') | int }},{{ states('input_number.led_strip_g') | int }},{{ states('input_number.led_strip_b') | int }}]
… well, the content is perfect formed.