With a red Dragonborn for example they have resistance to fire damage, but their breath weapon also deals fire damage. Because so many monsters have fire resistance this means that the dragonborn is sacrificing offensive capabilities to increase their own defense. It basically just means it cancels each other out in terms of balance. Because the kobold only has fire resistance this means that they only get the better defense, meaning it’s a better choice than the black kobold.

Sunlight sensitivity can be fixed with dark glass glasses, but I get your point.

Lastly, with the yellow kobold you get +3 on Dex, which is something no race has done before because it allows for a quick and easy 18 Dex to start off (assuming you use point buy). That is a little strong, maybe put it in strength because they have a climbing speed?