If it doesn't look like blood, than do not be too worried. However maybe your dog had eaten something out in the yard, example being some type of plant etc... that would cause this coloration in the stool, or the bright red spots you had mentioned. Did you or someone else dump any old table scraps outside. I know that sometimes this happens and dogs will eat anything dumped within range of their sniffer and gobble it up. If your dog appears to be acting normal and does not appear to be in pain to add, than I think you should be okay until monday to get him or her to the vets.In addition make sure your dog drinks plenty of water in the mean time, so it can flush anything in its system that it may have eaten. Hope some of my suggestions help you out.