Before entering a fortified hobgoblin camp, the party bard cast Seeming on the whole group to make them all appear as hobgoblins, including herself.
Later, but still within the eight hour duration of the Seeming, she wanted to use her Hat of Disguise to change her own appearance away from that of a hobgoblin. Hat of Disguise allows her to cast Disguise Self at will. Seeming is not a concentration spell and at the time I may have believed that it made no provision for the caster ending it [my error, as pointed out by Wyrmwood]. Thus there was an apparent conflict between the effects of the Seeming vs. that of the Disguise Self. For the purposes of answering this question according to RAW, we can assume that the bard chose not to end the Seeming because she didn't want to end the effect on the rest of the party, just on herself.
The PHB rule for "Combining Magic Effects" says
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
Seeming and Disguise Self are not the same spell cast multiple times and thus we should 'add their effects together'. But since one effect is attempting to make the bard look like a hobgoblin and another is attempting to make her look not like a hobgoblin, it is difficult to determine how to 'add these effects together'.
Even though Seeming and Disguise Self are not the same spell, the descriptions of the spell effects themselves are nearly identical: Seeming says it
allows you to change the appearance of any number of creatures that you can see within range. You give each target you choose a new, illusory appearance...The spell disguises physical appearance as well as clothing, armor, weapons, and equipment. You can make each creature seem 1 foot shorter or taller and appear thin, fat, or in between. You can't change a target's body type, so you must choose a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.
Disguise Self says
You make yourself--including your clothing, armor, weapons, and other belongings on your person--look different until the spell ends or until you use your action to dismiss it. You can seem 1 foot shorter or taller and can appear thin, fat, or in between. You can't change your body type, so you must adopt a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.
At the time, as DM, I ruled, 'if I can't tell any functional difference between these spell effects, I am going to treat them as if they were the same spell effect. Since there is no clear guidance on what 'most potent' means, I will then apply the errata rule for combining magical effects that the more recently-cast takes precedence.' I allowed the bard to change her appearance to what she wanted with the Disguise Self.
I am comfortable with my decision as a DM, but wondering if there is any RAW guidance for when there are two non-identical spells with contradictory effects that are to be 'added together'?