I would suggest against spray painting the cables. Though rubber paint might be flexible, the surface of an insulated wire is a terrible surface for any paint to adhere. If sleeving every individual wire is tedious, sleeve the entire stack of wires instead. Try looking for sleevings that are slightly larger than the common size.
If all else fails, you'd have to fork out for a pre-sleeved PSU. Fully sleeved aren't that expensive, for example Corsair CX500. More expensive ones will come with tighter weave sleeving, making it totally black.
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