These two cases are very different. Ethiopia did manage to stay independent, for various reasons best explained by an expert in the subject, but Liberia as a nation was essentially created by the United States as a place to repatriate manumitted slaves. The capital, Monrovia, is named after the US president, and the national flag is reminiscent of the American flag, both for this reason. The influx of American former slaves into Liberia had very serious consequences for the nation and the people there as well, as this new ethnic group, called Amero-Liberians, dominated the new nation's power structure for most of its existence, often at the expense of the peoples who had already been residing there.

Also as a side note, though it survived the so-called Scramble for Africa, Ethiopia did eventually succumb to European imperialism in the form of Mussolini's fascist Italy.

EDIT: I want to clarify that it wasn't the United States government that created Liberia, though a number of politicians supported the project, but rather independent groups in the United States.