I've been using Printful for quite a bit which is "decent" at best. I created a neon green vector image for a hoodie but when i received it in the mail it was a dull grass green.
When i reached out to them they said that they can't print that vivid (even though their print previewer showed they do). When i look online at other DTG services it appears that this isn't possible, however, looking at shirts from a company called Retro Rifle it DOES appear that it's possible to print neon colors: https://retro-rifle.com/collections/t-shirts/products/uzitee
TLDR; Looking for a print-on-demand service that can print neon colors on shirts.