The video posted on the Chinese mobile app Douyin (Tik Tok) and the article published by Liangshan News disproves the statement.
By reverse image search, the same video was found on the Chinese mobile app Douyin (Chinese version of the video-sharing app TikTok) with a caption that #Rainbow making machine.
As per the fact check done by the AFP, the Liangshan News, a local newspaper published an article on 21 March 2019 which shows a similar rainbow sighting photograph in its article. It had reported that in March, three additional dust control trucks were purchased. Every morning, afternoon and evening, a total of five dust control vehicles sprayed water on the main streets for dust control purposes, and it is also called 'aircraft artillery sprinkler'.
Therefore, the video showing a truck spraying in China's Sichuan province was for dust control purpose in March 2019, which was before the novel coronavirus outbreak.
The COVID-19 pandemic has given rise to a lot of potentially dangerous misinformation. For reliable advice on COVID-19 including symptoms, prevention and available treatment, please refer to the World Health Organisation or your national healthcare authority.