Why not yellow? (No note)...
Without getting into a lot of botany, there are -- speaking very broadly -- two main types of yellow onions: the sweet/sweetish as exemplified by certain Vidalias, and the "hot" (which they aren't; no capsaicin or equivalent). Either of these 2 properties tends to overwhelm the "onion" flavour in cold dishes.
White onions, contrarily, have a considerably more concentrated (I don't have the proper adjective here, sorry) onion flavour, with fewer distractions or sidelights. Hence, in this dish at least, the strong preference for white onions, esp. in my case, Bermudas.
Clearly, in any given dish, one can use any set or subset of ingredients that one wishes. In Gradma D's recipe (I have a fax of a Xerox of her hand-written recipe that pre-dates the Depression), she underlined "white" twice. Never understood why until I took Chem 43b, Chemistry of Food, in college.
I can promise you faithfully that nothing but white onions have ever appeared in her Mexican salad since, at minimum, 1956.
Of course, this preference might also have something to do with the colours of the Mexican flag. The onions don't turn colour when marinaded in white vinegar and refrigerated. This is just a guess on my part, though.
Happy T-Day to you!