colour plain tex keywords

[michael-franzese]

I have made the following definition to colour plain tex keywords
for greek letters \alpha \beta \gamma.

How can I associate the actual keywords wtih the face, so I can highlight
the lower case greek letter commands in plain tex.

(defface colour-greek-category
  '((default :inherit bold)
    (((class color) (min-colors 88) (background light))
     :foreground "#1f0f6f")
    (((class color) (min-colors 88) (background dark))
     :foreground "#92baff")
    (t :inherit font-lock-builtin-face))
  "Face for header category entry.")