Each road marker color or color combination has a specific purpose designated to it by the MUTCD (manual of uniform traffic control devices).
The white/red or yellow/red two way markers are used on one way roads where the driver going the wrong way would see the red and the driver going the correct way would see either white or yellow. In some cases, solid yellow, solid white, yellow/red or white/red markers are used to replace painted lines.
Red two way markers are used to mark areas where cars are not to go and the green two way markers mark areas where it is ok for cars to go. Much like a red or green light. Whereas the two way white/red markers allow traffic to go one way but not the other, the red markers would block both directions. The green would conversely allow traffic in either direction. For example, if you wanted to keep all cars out of an area in your facility that was for pedestrians only you could line any entranced with the red markers and also put up signs on both sides of the entrance. There are countless other applications for these two colors.
Finally,the blue markers are used all over the country to mark fire hydrant locations. These are placed either in the center of the road or at the edge. If a hydrant is at a corner then two markers are used. One for each street. Because of their universal application for hydrants it would be wise to avoid using the blue for any other application as this can confuse fire crews.
In private parking lots the blue, red and green markers generally serve the same purpose as on roads. For example, the blue would still always be used to mark a fire hydrant. The red may be used to highlight a fire lane or block off areas where cars are not to go. The green markers would do the opposite. White and yellow are often used in parking lots to designate cart return areas or to mark speed bumps. Road markers are normally not used in pedestrian areas due to the trip hazard they may cause. With this in mind markers are often used on factory floors to mark where pallets of material are to be stacked. In this case they are always out of the way of pedestrian traffic and are placed for fork lift traffic to see. The different color markers are available on this site and also on www.colebrothers.com .