You can’t possibly know what every ingredient in every chemical and artificial flavour/colour loaded candy bar is, and as such are not qualified to make a decision on its kashrus status based simply on the listed ingredients. Furthermore, many candy bars et al. usually list something along the lines of “and other natural flavours” as the last ingredient listing. That moniker is used to inform the consumer that there are ingredients in the item whose quantities are not large enough to require itemized listing, but are not necessarily batul b’shishim. Even if it was less than a sixtieth, it would still be debatable, and most would paskan assur anyway.
I could hear the moreh heter to self-paskan based on reading ingredients back in the 50’s, but in 2012, with the dearth of OU (et al.) certified products, to still self-paskan by reading ingredients shows a certain disregard for kashrus. Where no other option is available the case can be made, but if you can just as easily pick up the next, kosher certified candy bar and you don’t…something’s up.