How can I color part of Slack incoming-webhook messages?

I want to compose a python script which sends a color-formatted message to a configured slack channel. I managed to send sentences of plain texts to slack channels but I could not figure out how to confer part of the sentences colors that I want.

I used slackweb library in python as below

import slackwebslack = slackweb.Slack(url = url_of_the_configured_channel)string_to_send = "test TEST"payload = {'text' : string_to_send}slack.notify(text=string_to_send)

how can I, say, format string_to_send or payload so that the word 'test' appears blue and 'TEST' appears red in the slack channel?

Thanks in advance.