Nothing in those photos implied a white core. White-painted composite sections, but not white on the actual core. The cartoon was white, but lots of cartoon depictions of SLS have been white. I'm still betting against it. Painted SOFI (spray-on foam insulation) is just heavy and expensive. Reference the Shuttle Program with one painted ET (or was it two?). The only engineering reason I can think of for doing it would be to provide a coating better suited to mitigating electrostatic discharge. So, I suppose there's a chance!

For what it's worth, SOFI is pale yellow when first applied. It gradually turns more and more orange - almost brown - when exposed to UV light. Bond-on foam insulation (BOFI) maintains a more yellow appearance. Looking at the Green Run booster, I think there's quite a bit of BOFI on there too.