Does your baby have a red bum? The following presents many of the common scenarios. From my experience, 95% of rashes are caused by something correctable in the wash routine - either switching to a different detergent, adding an extra rinse, extending the first cold rinse to a full cold wash. The other 5% are something systemic with baby - yeast, bacteria, teething, food sensitivity, etc.

DISCLAIMER: This page is not intended in any way, shape or form to stand-in for a medical professional's opinion. It is solely to give some ideas as to whether the problem is something in the diapers, or something in baby's system. We always recommend consulting with your pediatrician and defer to your pediatrician's diagnosis and treatment plan. 

First, consider the location of the redness. This will help pinpoint if the redness is caused by the diaper itself, or by something external to the diapers.

Across baby's buttocks, primarily on the surface of the cheeks
If baby spends quite a bit of time on his/her back, this might be a sign of detergent buildup in the diapers. Reason being, the lingering detergent in the diapers is 'reactivated' by baby's urine. And a baby on his/her back will have the most urine pooling against the bum, so the most concentrated/wet fabric is against baby's cheeks. Diaper Fix: If this is the case, the diapers need to be stripped. If the redness comes back, you should consider switching detergents (particularly to something free of enzymes).

If this appears in an older baby or toddler (in other words, a baby sitting or standing most of the day), the redness is likely brought on by strong poop sitting against the skin. Pay attention to baby's diet, if you introduced any new foods. In addition, watch for signs of new teeth.

Redness primarily across the scrotum or labia.
Again, what is baby's position? If baby is spending quite a bit of time on his/her stomach, or is an older baby likely leaning over or standing most of the day, this redness could mimic what is happening above - with urine being the most concentrated in these areas and 'reactivating' residual detergent.
Diaper Fix: Strip the diapers to thoroughly rinse any remaining detergent.

Between baby's cheeks, a ring of redness surrounding the anus.
This most often indicates a food sensitivity, particularly dairy. Discuss the possibility and other ideas with your pediatrician.

Raised, red bumps scattered through the diaper region.
This can indicate several things - irritated eczema, heat rash, yeast or another infection. I recommend a visit to the pediatrician to determine what it is.
Diaper Fix: For an infection, you'll want to sanitize the diapers. If it is heat rash that seems to never resolve, you should consider switching to a more breathable diaper (ie. fitteds worn coverless around the house or fitteds+wool).

Redness in the leg creases - approximately lines up with the leg casings of the diapers
As the elastics in the legs of diapers are some of the most dense fabric in a diaper (and therefore the hardest to rinse), this can sometimes be the spot detergent buildup presents first.
Diaper Fix: Stripping the diapers should take care of it.
If it persists despite thorough rinsing and the diapers don't ever seem to have a stink to them (another sign of detergent build-up), it can be a sign of fabric sensitivity, and a switch to another style of diaper is recommended.

Um, it looks like my baby is wearing a red diaper - aka - anywhere the diaper touched is bright red.
This usually corresponds to a fabric and/or moisture sensitivity. The most common sensitivities are: suedecloth, then poly fleece, then natural fibers (not the natural fibers themselves, rather, the moisture the natural fibers holds against the skin).
Unfortunately this one requires a bit of guess-and-check on your part, as you don't want to destash a bunch of diapers that might have something else going on.
Diaper Fix: Switch up your diapers for a few days. Using all pockets? Try laying a prefold in the pocket instead, so the cotton is against baby's skin. Using all bamboo fitteds? Try laying a poly fleece layer in the diaper, so there is a wicking layer against baby's skin.