Tubaloth was king of the Lamanites but he was not a Lamanite by birth. As Ammoron's son, he was of Nephite dissent. A common mistake is to assume that all the Lamanites were dark skinned and all Nephites were light skinned, but a careful review of Book of Mormon history indicates that there was all sorts of inter-racial mixing. This is particularly true at this time. The division between Nephite and Lamanite was based on religious grounds not racial ones. And apparently, there was no prohibition to a Nephite's ascendancy to the Lamanite throne.