Weird things sometimes happen to your paint. Spooky Halloween kind of things you never thought were possible...
If you're painting large, exterior broad-wall surfaces with paint tinted to moody, heavy or inky colours, then be aware that it takes a while for the coating to completely cure. Water-based acrylics actually take over a week to completely dry, so it's important that the dark surface does not come in to contact with any moisture during this time. Sometimes this can be almost impossible to avoid. Condensation, rain, or even a sprinkler can wet the surface which will cause surfactant staining.
Surfactant staining looks like an oily, soapy residue on the paint surface and is pretty unsightly. But, don't worry if this discolouration appears on your wall. Simply grab a hose, and with a slight finger pressure over the hose end, wash away the stain. It might take a couple of goes if the staining is quite strong, but the sooner you apply this technique the quicker your wall will be back to an even, consistent finish.
Easy, and if you have any questions please send me an email.