the primary camera on MGS is monochrome, not color. That's the hi-res portion of the camera. There is a good technical paper on the MOC here. The MOC has three parts - a narrow angle high resolution camera (monochrome) and two wide angle cameras that use filters to produce color images at lower resolution.

There are several limitations for the MGS that affect the number of images it can return. Starting at the MOC, there is a 12 megabyte buffer to hold and process the images. This memory is used to hold the unprocessed data, provide for compression, and store the compressed image before transmission. This limits how many images can the taken on any given orbit. At the highest resolution (1.4 meters per pixel), a 2.8 KM x 2.8 KM uncompressed image at 1 byte per pixel takes 4 megabytes. The system then needs some working room to compress the image before discarding the uncompressed image.

Another limitation is the data rate that can be transmitted from Mars to the Earth. This can vary depending on the distance between the two planets. According to some sources, the maximum data rate is 85,333 bits per second. However, it's not always be possible to achieve that data rate depending on the signal-to-noise ratio. This can be reduced by greater distance between transmitter and receiver some times during the year (like right now) as well as an increased background noise level due to things like solar activity.

From this source:

"How fast does Surveyor transmit data to Earth?

The answer depends on Surveyor's distance to the Earth. When Mars is farthest from the Earth during mapping operations (summer 1998), the maximum data rate will drop as low as 21,333 bits per second. When Mars is closest to the Earth (summer 1999), Surveyor will use a data rate of 85,333 bits per second. At times in between those two extremes, Surveyor will use a medium data rate of 42,667 bits per second. In contrast, modems on home computers transmit data at an average speed of 28,800 bits per second."

If the image memory buffer were full (12 MB), it would take over 4700 seconds (1.3 hours) to transmit at the lowest data rate. Unfortunately, that's more than the maximum window where MGS can transmit to Earth so you're limited in how many images you can take in an orbit. Actually, that's an unrealistic scenario because they can't fully fill the memory and still process images. Since it takes 4 megabytes to hold a single uncompressed image and there is some system overhead, you can only capture a small number of images per orbit.