@AlwaysCurious
In your example, both the fish and vegetables would be called “ulam”.
Can you say 'Yun lang ba ang ulam mo? Baka magugutum ka pa mamaya.'? or, because there is no real protein in potato chips, should you say 'Yun lang ba ang kinain mo? Baka magugutum ka pa mamaya.'
It will only be valid to say " 'Yan lang ba ang ulam mo? " if you also see rice there. Without the rice, it should be " 'Yan lang ba ang kakainin mo? " (Is that all that you'd eat?).
“ ‘Yun ” (short for “iyon”) is “there yonder” or if you’re referring to something that you do not see at that moment. For the potato chips, if you are talking to someone who is holding potato chips, then you have to use “ ‘yan “ (short for “iyan” - that, which is near to you both).
It would sound more natural as “Baka magutom/gutumin ka mamaya”. You also don’t really need the “pa” (still, yet). You use the infinitive because “baka” (maybe, might) makes the phrase subjunctive. “Magutom” is subject-focused and “gutumin” is object-focused. “Gutumin” can be used here without an object because there can only be one object being referred to and is, therefore, already understood as “the absence or lack of food”.
“Baka magugutom/gugutumin ka pa mamaya” = Maybe you will be hungry still later.
Better: “Baka magutom/gutumin ka (uli) mamaya” = You might be hungry (again) later.
“ ’Yun lang ba ang kinain mo?” = Was that all that you ate? (“yun” would be all right because what was eaten is no longer visible)
“ ‘Yun lang ba ang kakainin mo?” = Is that all that you will eat? (referring to a food item not seen at that moment)
“ ‘Yan lang ba ang kakainin mo?” = Is that all that you will eat? (the food is seen by both of them)
“Pagkain” is “food”, which includes rice. “Ulam” is everything that can be eaten with rice or bread, if it is going to be eaten as a substitute for rice.
As mentioned by FilipinoChatAdmin, “...ulam is something served with rice, so meryenda or potato chips eaten alone would not be considered ulam...”. However, if you’d eat potato chips along with rice, then they will already be called “ulam”.