Weight, altitude, and temperature all contribute to what cruise speed the airplane is capable of flying on a particular day. The wing is designed to be most efficient at a particular range of speeds. Older jets, and those designed for long-haul flying, will have wings designed for somewhat higher cruise speeds. Usually M.80-M.85. Shorter range jets will typically be in the M.75-M.80 range.
As you go higher, or temperature increases, the range of speed choices narrows. Vmo/Mmo is our equivalent to a piston airline's redline though it is never higher than what would be the top of your green arc (Vno). We call it the barber pole as it is a red/white striped needle. The barber pole moves as conditions change so at lower altitudes it will represent Vmo (max speed in KIAS) and at higher altitudes it will represent Mmo (max speed in Mach). Our low-speed buffet (beginning of a stall) speed (KIAS) increases dramatical at altitude and will often be over 200 KIAS. Our Mmo is based, among other things, on our high-speed buffet (high-speed stall from the airflow separation due to the transonic flow over the wing). If we climb too high (for the wing) there isn't enough room between the low and high-speed buffets to have a comfortable margin. This is why on longer flights we'll start out at a lower altitude then step-climb throughout the flight as fuel is burned off.
The speed that we actually fly, within the range of available speeds, will vary based on a cost index. The cost index is entered into the FMS and represents the operator's preference for speed vs. economy. If a flight is running late, or into stronger headwinds, a higher cost index is used which results in a higher cruise mach and higher fuel burn. The operator can also adjust the cost index based on the cost of fuel vs. the other direct operating costs of the flight. This would result in higher speeds when fuel is relatively cheap, to save on maintenance costs, and lower speeds when fuel is expensive, to save on fuel cost.
In the B737 we typically fly around M.78 but are usually capable of M.80. In the B767 we were usually no less than M.80 and sometimes as high as M.82. The DC8 didn't have an FMS to do a cost index calculation. We used M.80 as a long-range cruise and M.82 as a high-speed cruise. The E145 that I commute to/from work on usually comes up with an economy cruise speed around M.67 on the FMS but they typically fly around M.76-M.78.
