I want to override highlighting for a language-specific keyword in Neovim.

I’ve figured I can do it via vim api (eg. vim.api.nvim_set_hl). This approach does work with elements like Comment or LineNr, but doesn’t seem to work with rubyInstanceVariable.

:checkhealth says Ruby parser is installed:

Parser/Features        H L F I J
 - ruby                ✓ ✓ ✓ ✓ ✓

colors.lua

   function ColorMyPencils(color)
        color =  color or "default-color-scheme"
        vim.api.nvim_create_autocmd("ColorScheme", {
            pattern = "*",
            callback = function()
                -- This works
                vim.api.nvim_set_hl(0, "Comment", { fg = "#818589" })
                vim.api.nvim_set_hl(0, "LineNr", { fg = "#818589" })

                -- This doesn't work
                vim.api.nvim_set_hl(0, "rubyInstanceVariable", { fg = "#818589" })
            end,
        })
        vim.cmd.colorscheme(color)
    end
    
    
    return {
        {
            require("zeropoop/my-color-scheme").setup({}),
            ColorMyPencils("my-color-scheme")
        }
    }

I’m just starting with Vim and Lua and will appreciate any advice.