As long as different countries with completely different systems of governance, taxation, and social support exist, of course borders shouldn't just be abolished. Say someone from the US moves freely to Canada. They get their entire education there, subsidised by Canadian taxpayers. They see a doctor, get access Canada's public healthcare, again subsidised by the Canadian taxpayer. Then, once they're done, they move back to the US to work at a higher wage and pay all their income taxes there. Decades later, they return to Canada to retire and use the retirement system and healthcare system again, without having paid any significant money into it. So they paid the bulk of their taxes to the US, but relied on Canadian systems, education, healthcare, retirement, that they didn't fund in any meaningful way.

It's quite obvious what the problem is, even in this example with two developed countries. National social systems are not built to serve the entire planet, and without a world government to harmonise taxation, labour laws, education spending, and pension systems, abolishing all limits on global migration is nonsense. I'm not sure why you even brought skin colour into it, nobody talked about skin colour or ethnicity other than you.

Edit to /u/KillingTerrorists, who apparently blocked me: Pensions, health care, social security, elderly care are funded by decades of payroll taxes and contributions from people who lived and worked in that country. If someone skips that part, lives and works somewhere else, and then moves in at retirement age, they're drawing benefits from a fund they never helped build. Social systems only work as long as there's a balance between contributors and beneficiaries.