Verbed color names and "-en"

"whitened", "blackened", and "reddened"; but "yellowed", "grayed", and "blued".

Is there some rule or is it just one of those things?

"Greened" makes sense; no one is going to say "greenened".

"Pinked" or "pinkened"?

"Purpled" or "empurpled"?

First person to not make an Arrested Development joke gets a +1 from me.

It looks like words that end with a hard consonant get the "-en".

This answer concerns only the colours pink and purple

Pink Pink ends on a hard consonant thus "to pinken" would seem logical and the Merriam-Webster indeed lists this as a legitimate form but ONLY as an intransitive verb. According to the Merriam-Webster, the form "to pink" means many things but NOT "to make something pink" . Oxford Dictionaries, however, does list "to pink" as relating to colour but ONLY as an intransitive verb.

Strangely enough, none of the dictionaries seem to list a transitive form, which suggests that we should use "to make pink" for this purpose. However, personally (and I think many others with me) would be perfectly comfortable to use "to pinken" as a transitive verb - even though the dictionaries don't agree with me.

Purple Both "to purple" and "to empurple" are valid forms.

The explanation is more likely to be semantic and morphological than phonetic. The OED records as both transitive and intransitive verbs black (and blacken), white (and whiten), purple (and empurple) green, pink, yellow, pink, grey, and blue, although not all are in current use. The forms white, black and purple first occur either at roughly the same time as the longer forms, or predate them. The –en suffix is found in verbs other than those which indicate a change of colour. The OED begins it etymological note on the suffix with:

Most of the words of this type seem to have been formed in late Middle English or early modern English, on the analogy of a few verbs which came down from Old English or were adopted from Old Norse . . .’