There is a Supreme Court ruling that IQ tests are assumed to unfairly disadvantage minority candidates. This was in the context of employment rather than college admissions, but I don't think anyone has been bold enough to test whether it applies.

Broadly speaking, IQ tests tend to include a lot of cultural assumptions and therefore members of the in-group will test higher than members of out-groups of the same aptitude. IQ tests are therefore treated as discriminatory (disproportionate impact) "by default." The administration of a particular test for a specific purpose can usually be OK'd by either showing that particular test is not discriminatory, or by showing that particular test has a measurable correlation to the specific purpose for which it's used. My understanding is that "specific purpose" in the employment context means the individual job description, not just hiring in general.