We, the US military, use a couple of types of sand bags. Some are the traditional burlap that come in either light olive green or natural burlap color (tan) or any shade in between. We also have some fake nylon burlap bags that come in a light olive color or desert tan. These are shiny and not as good as the traditional burlap ones.
Sand bags are fairly porous and tend to take on the color of the sand inside. This is because all the minute particles of dust get squeezed out of the bags as they are stacked, filled, tied, etc. Now if you fill them with dirt like when you are in woodlands, they tend not to change color as rapidly as sand.