E254: Cannot allocate color 235

[shane.qian]

sometime it happened such E254 err at my v9.1.698 (linux xfce-term xterm-256color)

Error detected while processing ColorSchemePre Autocommands for "*":
E254: Cannot allocate color 235

```
hi clear difftext
hi difftext cterm=bold ctermbg=red ctermfg=235

au colorschemepre * if has('termguicolors') | set termguicolors | endif
```

but mostly it probably cannot be simply reproduced, so just asking anyone knew what happened?

--
shane.xb.qian