Neovim colour scheme in Urxvt not rendering correctly

Question

I'm trying to use a 256 color theme in Neovim (this one: https://github.com/ayu-theme/ayu-vim), but can't get it to render correctly on Urxvt, even though it seems to support 256 colors:

Here is the relevant portion of my .vimrc:

Plugin 'ayu-theme/ayu-vim'
...
syntax enable
set termguicolors
let ayucolor="light"
colorscheme ayu

The color scheme works in Neovim on XTerm and in gvim, making me suspect it's an issue with Urxvt. I've tested this with and without modifications to Xresources.

I'm on Ubuntu with the rxvt-unicode and rxvt-unicode-256color packages installed.

Answer 1

This turned out to be me misunderstanding what the termguicolors option entails in Vim. The ayu colour theme actually requires Truecolor support, which Urxvt does not have.

There is a good explanation of colour support in terminals: https://gist.github.com/XVilka/8346728

This has finally given me a good enough reason to switch to Alacritty.