Aristotle refers to letters as the cause (his word aitia) of a syllable in Metaphysics BK 1 and Physics book 2 part 3. He doesn't specifically say material cause (but you're not the first to draw the inference by a long shot).
I'm not entirely following the dichotomy you propose but I would suggest the following interpretation I take to be Aristotleian.
In general, letters are the causes of syllables insofar as need the letters to say the syllables that are the words. (This may be bad linguistics but give a 2500 year old scientist a break). I take him to mean by syllable a type of uttered sounded made up of the letters in the alphabet.
In your example, you seem to be looking at two samples of [digital] writing in different colors and asking whether he sees syllable as a concrete instance or abstract type.
I think there's two different things going on that should be looked at separately here and mixing them makes the difficulties. First, there's a computer image [or if you prefer physical piece of paper]. Second, there's words made of letters.
- Actual material causes themselves are by definition non-abstract.
- Actual things are things that exist.
I would say green pixels [or ink or graphite] and blue pixels [or ink or graphite] respectively are the material causes of readable words within images [or on the sheet of paper]. These pixels (plus that background color of pixels) are the images material cause.
At the same time, there's a word "bam". the writing of b, a, and m as letters are the material causes necessary for the existence of a syllable "bam." But blue ink or green ink respectively is necessary for the existence of a readable image as an existing thing.
Are the written syllable and readable image identical? Not in every single respect -- as is clearly demonstrated in that we can produce the same syllable differently. But that's just a feature of the general discernability of materially different things.
So I take it that no material cause is an abstraction. But there can be material causes of different things (words vs. images) in the same overall object.