You mention its been outdoors. have you had any cooler nights? It looks fine to me, just still a little damp.

These need full sun- either outside in full sun w/ no shade or in a south-facing window, possibly w/ a grow light as well. If you have it potted up in soil & sand, and you do not live in a very arid or desert climate climate, then your soil is going to hold water way too long & will rot this plant. Is the plant squishy & soft where the yellowed areas are? if so, it may already be rot. Then I would cut cactus off (roots stem & all) where the rot ends & you see only green flesh- a clean, even slice. Lay it out in a somewhat shady spot to callous the cut over about a week. Mix about 70-80% of pumice, chicken grit or sifted, coarse-grade perlit into 20-30% soil, then set calloused cactus in top, put outside in sun or inside right up in a south window, & wait for it to root. This could take months, even up to a year & w/ winter coming, keep in mind that all growth slows considerably. While its unpotted, If the yellowed areas are not mushy, , look for brown, mushy and/or foul smelling roots. If you find those, (or any dried out roots) clip those off. leave the cactus out for a few days, unpotted, to dry out, again again, shaded spot. Then mixup same soil combo I mentioned above, repot & put in same spots mentioned above.

If neither the cactus or any roots are rotten, I would still repot & move into same soil combo & location mentioned above to avoid rot from happening.

I agree with Rebecca, but a plant without roots should be kept out of direct sun until the roots are well established. In the sun even a cactus will dry out without roots.