Tandy 1000 HX Technical Reference Manual page 401

In black and white modes:
BL = 0: 1 for white
BL = 1: 1 for black
In 4 color graphics modes:
BL = 0 (1 = green / 2 = red / 3 = yellow)
BL = 1 (1 = cyan / 2 = magenta / 3 = white)
In 16 color graphics modes:
(1 = blue / 2 = green / 3 = cyan / 4 = red / 5 = magenta
/ 6 = yellow / 7 = light gray / 8 = dark gray / 9 = light
blue / 10 = light green / 11 = light cyan / 12 = light red /
13 = light magenta / 14 = yellow / 15 = white)
Note: For alpha modes palette entry 0 indicates the border color.
For graphics mode palette entry 0 indicates the border and the
background color.
Write Dot
Write a pixel (dot).
Entry Conditions:
AH = 12
DX = row number
CX = column number
AL = color value (When bit 7 of AL is set, the resultant color
value of the dot is the exclusive OR of the current dot color
value and the value in AL.)
Read Dot
Read a pixel (dot).
Entry Conditions:
AH = 13
DX = row number
CX = column number
Exit Conditions:
AL = color value of dot read
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