I guess I was a little obsessed with color when Miner was programmed. All of my prior games were written for the Radio Shack TRS-80 model I which had fairly limited monochrome graphics.
The high score display screen is a good example of this obsession:
I wanted to pack as much color as possible into this screen; in fact I wanted more color than was possible on the Atari. If you look close you'll see at least 15 different colors on the screen at any one time. There were several tricks I used to achieve this.
The top line has four "5"'s on the left that are playfield registers. These move slowly toward the right using horizontal scrolling until they have reached the next character position. At that point they are reset to the left and the color registers are modified for the next incoming character. The four "5"'s on the right side of the playfield are also playfield registers being manipulated in the same way. Tight code timing is employed to change the color registers in the middle of each scan line to allow different colors on the right side of the screen. The two remaining "5"'s in the center of the top line are players.
A similar method is employed on the bottom line to move those characters to the left.
The upward characters on the left side of the screen are a single player. The color register for this player is altered several times to allow multiple colors from this single player. The right side uses the same technique.
The rainbow effect on the letters in the background is simply one color register being modified each scan line.
In the game, Bounty Bob is made up from three players:
Player zero is blue, player one is red-orange, and player two is white. When players zero and one are overlapped on the Atari the color of the pixels common to both is the logical "or" of the two color registers. Blue ($8A) or'ed with red-orange ($3A) equals green-blue ($BA).
When Bounty Bob explodes after hitting an alien the size registers are being changed from X1 to X2 to X4 rapidly to achieve the exploding effect.
The aliens use the remaining two players (one is actually the four combined missiles). Display list interrupts are used to get more than just two aliens on the screen by changing the color and horizontal position registers several times throughout the screen. Their glowing appearance is just the color register being cycled from dark to light and back.
The white lines in the picture show where the display list interrupts occur.
At the top of the screen is a mode "2" text line for the scores, and the remaining lines are mode "e" graphics lines. Mode "e" allows for three playfield colors plus the background color.