Here's the plan

We want to be able to map any RGB color to an approximate "256 color"
In general, the 256 colors have three zones:

Zone 1 - Legacy colors (8 basic colors and 8 "bright" colors -- 16 colors)
Zone 2 - 6-bit RGB space (6x6x6 == 216 colors)
Zone 3 - Grayscale (24 colors)

ZONE 1
======
Zone 1, the legacy colors, are too varied to be mapped reliably.
So we won't be mapping any colors to it.

ZONE 2
======
In Zone 2, every color is represented as an RGB value with 6 values
rather than 256 values.  To convert 256 values to 6 requires lossy
compression -- and the bands to be used are somewhat speculative.
I'm going to use the xterm bands

	Value	Normative	Band
	------	----------	-----------
	  0	   0		  0 -  47
	  1	  95 (5f)	 48 - 114
	  2	 135 (87)	115 - 154
	  3	 175 (af)	155 - 194
	  4	 215 (d7)	195 - 234
	  5	 255 (ff)	235 - 255

Note there are 6 zone 2 colors that map to "grayscale colors" (which
will be discmissed more later)
	color	rgb tuple
	-----	--------
	 16	  0
	 59	 95
	102	135
	145	175
	188	215
	231	255


ZONE 3
======
In Zone 3, every number has RGB values approximately equal,
with 24 values starting with 8 + (10 * value) (to 238)
interposed with the 6 "gray" colors called out above.

A color is "approximately equal" if the largest difference
between the RGB values is 10, they are declared "equal"

Colors where some pair of RGB values are more than 10 apart,
then they will be mapped to a zone 2 color.

The "value" of a zone 3 color is (R+G+B/3)

	Value	 Normative	 Band
	-----	-----------	------------
	<16>	   0		  0-4
	 0 (232)   8		  5-13
	 1 (233)  18		 14-23
	 2 (234)  28		 24-33
	 3 (235)  38		 34-43
	 4 (236)  48		 44-53
	 5 (237)  58		 54-63
	 6 (238)  68		 64-73
	 7 (239)  78		 74-83
	 8 (240)  88		 84-91
	<59>	  95		 92-96
	 9 (241)  98		 97-103
	10 (242) 108		104-113
	11 (243) 118		114-123
	12 (244) 128		124-131
	<102>	 135		132-136
	13 (245) 138		137-143
	14 (246) 148		144-153
	15 (247) 158		154-163
	16 (248) 168		164-171
	<145>	 175		172-176
	17 (249) 178		177-183
	18 (250) 188		184-193
	19 (251) 198		194-203
	20 (252) 208		204-211
	<188>	 215		212-216
	21 (253) 218		217-223
	22 (254) 228		224-233
	23 (255) 238		234-246
	<231>	 255		247-255