you should add coolant if the level is lower than minimum. Normally the
coolant level should not drop as it is a closed system so theoretically it
should never get out. Small leaks can cause you to occasionally loose
coolant. It's ok to drive with the level below max but try to check it and
fill as necessary if it does leak out.

The worst possible senario is that you have a cracked head and that would be
bad to drive the vehicle that way. You can check this by looking at the oil
dipstice to see if any coolant has entered the oil pan. The oil on the
dipstick will have a white-milky color which is bad for the motor, don't
drive it if you find this, get it fixed as soon as possible. This is not too
common if you keep the maintenance up on the vehicle. More common problems
are a bad radiator cam, leaky hose or water pump. I just didn't want you to
drive it if there is a serious problem. If you are not sure its best to get
it checked out before you go on a long drive

The color changes as it picks up contamination from the system. Most coolant
is a yellow-green color new but turns a dark green almost brown color as it
sits in the system. If you fill the overflow bottle to the high line you
should be ok to drive until you can get it flushed. You might want to have
the system pressure tested by a mechanic if you frequently have to add
coolant, this indicates a possible problem. The sooner you find it, the
cheaper (usually) it is to fix.

Hope that helps