suffix
Dutch compounds often contain interfixes, e. g. "-s-" and "-en-" in schaap+s+hond (sheepdog) and schaap+en+kaas (sheep cheese).
I think the -y in greeny-grey is better regarded as a type of interfix rather than an infix, simply because infixes must have a distinct paraphrasable meaning.
The term interfix was first coined by the University of California's Yakov Malkiel for an infix devoid of meaning.

SMART Vocabulary: related words and phrases

Linguistics: morphology & parts of words
-athon
affix
affixation
agglutinate
agglutinative
analytic
contraction
ion
ism
libfix
monomorphemic
monosyllabic
morpheme
morphological
suffix
syllabify
synthetic
that'd
that'll
that's