How to change text color at arbitrary cursor position programmatically?

[cancandan]

is this possible?

[ghost]

You will need to talk to Neovim directly, using document.getElementById('my-editor').editor.getClient() if I'm correct. For a full reference of what it can do, see https://github.com/rhysd/promised-neovim-client/blob/promisified/index.d.ts. I think you are looking for setCursor.