"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.
There seems to be a fair amount of literature agreeing with JeffSahol's statement that the use of -en is related to the sound of the word. According to "Argument Structure and Morphology: The Case of en- Prefixation Revisited", by Susanna Padrosa Trias (ASJU, XLI-2, 2007, 225-266):
a number of authors (e.g. Halle 1973, Aronoff 1976, Siegel 1979, Scalise 1984, Fabb 1988) have noted [the existence of verbs like redden] and have claimed that -en is the element triggering the conversion of As to Vs. [...] I suggest that there is a single suffix which sometimes has phonological content (the -en morpheme) and sometimes does not (the zero-suffix), thus avoiding the unwanted double forms for a unique function. The aforementioned authors agree that there are some constraints on the suffix’s attachment. The suffix -en only attaches to monosyllabic As that on the surface end in a single obstruent, preceded by a vowel, which optionally may, in turn, be preceded by a sonorant. If an A violates the condition just stated and there is a related nominal form that satisfies it, then -en attaches to the N: e.g. frightenV (afraidA has two syllables), and strengthenV/lengthenV (strongA and longA end in a nasal).
(p 244)
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 . . .’
