Could you please explain how to you 'some racist group' is equivalent to the Catalonian regional government, the democratically elected government of 7.5 million people?
Make it some major of a southern village, or even the whole State of Alabama, if you prefer. It would't make the question about Jim Crow laws more legitimate.
The legitimacy of 'Jim Crow laws' would be determined by how people voted - and presumably people would vote No. The 'legitimacy' of a referendum is related to who is trying to organise it - here it is the democratically elected government of 7.5 million people - that's why your initial 'some racist group' was a poor analogy.
Your analogy is totally invalid anyway. This vote is not equivalent to a call for racism, it's a call for self determination. The Catalonians have a fundamental moral right to self determination. From Wikipedia: The right of people to self-determination is a cardinal principle in modern international law (commonly regarded as a jus cogens rule), binding, as such, on the United Nations as authoritative interpretation of the Charter's norms. It states that a people, based on respect for the principle of equal rights and fair equality of opportunity, have the right to freely choose their sovereignty and international political status with no interference. ... self-determination entails the "moral double helix" of duality (personal right to align with a people, and the people's right to determine their politics) and mutuality (the right is as much the other's as the self’s). Thus, self-determination grants individuals the right to form "a people," which then has the right to establish an independent state, as long as they grant the same to all other individuals and peoples.
The UN have also been very clear specifically on the referendum that regardless of whatever regional or national laws there are, Spain MUST respect the fundamental rights (which trump constitutional or national laws because of their fundamental nature) of the Catalonians. The UN said '[Spain must ensure they] do not interfere with the fundamental rights to freedom of expression, assembly and association, and public participation [of the Catalonians].' They go on to be very clear "Regardless of the lawfulness of the referendum, the Spanish authorities have a responsibility to respect those rights that are essential to democratic societies”
So to be clear - according to the UN office of the high commissioner of human rights, it is the Spanish government who are acting illegally in violating the fundamental rights of the Catalonians.
Make it some major of a southern village, or even the whole State of Alabama, if you prefer. It would't make the question about Jim Crow laws more legitimate.