Monochrome Minifig availability chart

As of April 2016, there are 34 monochrome minifigures that can be completed with Lego parts.

Many of the parts can be purchased through BrickLink or BrickOwl -- the links are to my store, FigBits, which has many monochrome parts.

For absolute purists, 20 figures can be made using only parts that have been released in official Lego sets:
ID	BL ID	swatch	common name	official Lego name
26	11		Black	Black
23	7		Blue	Bright Blue
37	36		Bright Green	Bright Green
25	8		(Old) Brown	Earth Orange
26	199		Dark Bluish Gray	Dark Stone Gray
27	10		(Old) Dark Gray	Dark Gray
221	47		Dark Pink	Bright Purple
145	59		Dark Red	Dark Red
138	69		Dark Tan	Sand Yellow
28	6		Green	Dark Green
194	86		Light (Bluish) Gray	Medium Stone Gray
2	8		(Old) Light Gray	Gray
106	4		Orange	Bright Orange
26	115		Pearl Gold	Warm Gold
21	5		Red	Bright Red
192	88		Reddish Brown	Reddish Brown
135	55		Sand Blue	Sand Blue
5	2		Tan	Brick Yellow
1	1		White	White
24	3		Yellow	Bright Yellow
Another 4 figures are possible by using some minifigure heads that are available in unprinted version even though they haven't yet appeared that way in sets. These are still official, unaltered Lego parts, so most collectors would still consider these 5 colors as "pure" monochrome minifigs:
ID	BL ID	swatch	common name	official name
140	63		Dark Blue	Earth Blue
308	120		Dark Brown	Dark Brown
141	80		Dark Green	Earth Green
102	42		Medium Blue	Medium Blue
If you are willing to wipe the print off of a part, another 9 possible monochrome figures become possible:
ID	BL ID	swatch	common name	official name
212	105		Bright Light Blue	Light Royal Blue
221	28		Flesh	Nougat
119	34		Lime	Lime
250	71		Magenta	Bright Reddish Violet
322	156		Medium Azure	Medium Azur
312	150		Medium Dark Flesh	Medium Nougat
324	157		Medium Lavender	Medium Lavender
330	155		Olive Green	Olive Green
151	48		Sand Green	Sand Green
And finally if you are willing to accept "damaged" parts, you can use a disassembled Chrome Gold minifigure keychain, and replace the hands (though I wouldn't recommend this!) to get one more complete figure. There is also a Chrome Red minifigure keychain, but no Chrome Red hands are available.