I am by no means an expert on rain, but I found that you have to be OK with 'ruining' your painting. Go over it diagonally with the colors so they blur together a bit to give the impression of rain movement.

nostr:nevent1qqsdaflz0y7hqh3mj4d96d3msqev9v8964qu8xch69am7zsgyhm2u0gppemhxue69uhkummn9ekx7mp0qgsy6tv9w5faguux9ww6jhj3z7m5d34rz4dge7caftapjhns0hj4dfqrqsqqqqqptgf0xt

(Not mine, just another example)