Thanks for providing the quotes.
Still can't offer a definitive answer, but here's what I see.
substrates of high negative charge density
While not the textbook definition of a base, high negative charge densities are indicative of good nucleophiles - and potentially protic bases. The book has essentially defined the substrate to be non-acidic (although not necessarily basic).
The thing about Eosin is that it is itself an acid
Actually, it is a base. Check out that wikipedia page again. The phenolic O-
in the lower left and the carboxylate COO-
makes this a dibasic compound (probably as the disodium salt, although that is not made clear. Same with orange G. The two sulfate SO3-
groups make this also a dibasic compound. (It is also technically amphoteric, as the phenolic OH group can act as an acid with a pH ~10) Wikipedia does mention orange g is typically a disodium salt.
Based on your quotes, I'm going to stick to my answer. While I have not found anything unambiguous online, Eosin Y and Orange G are most definitely bases, and substrates of high negative charge density are unlikely to be strong acids, and more likely to be bases themselves (and thus will react with your cationic dye - an acid by itself).