Outlook 2011 category colors - Windows Color Palette?
Using /Applications/Utilities/DigitalColor Meter.app set to RGB as Actual Value, 8-bit and then hover over each color to find it's RGB color values.

Color Values:

## COLOR       HEX     (RRR GGG BBB)
 1 Red         #E7A1A2 (231 161 162)
 2 Orange      #F9BA89 (249 186 137)
 3 Peach       #F7DD8F (247 221 143)
 4 Yellow      #FCFA90 (252 250 144)
 5 Green       #78D168 (120 209 104)
 6 Teal        #9FDCC9 (159 220 201)
 7 Olive       #C6D2B0 (198 210 176)
 8 Blue        #9DB7E8 (157 183 232)
 9 Purple      #B5A1E2 (181 161 226)
10 Maroon      #daaec2 (218 174 194)
11 Steel       #dad9dc (218 217 220)
12 Dark Steel  #6b7994 (107 121 148)
13 Grey        #bfbfbf (191 191 191)
14 Dark Grey   #6f6f6f (111 111 111)
15 Black       #4f4f4f ( 79  79  79)
16 Dark Red    #c11a25 (193  26  37)
17 Dark Orange #e2620d (226  98  13)
18 Dark Peach  #c79930 (199 153  48)
19 Dark Yellow #b9b300 (185 179   0)
20 Dark Green  #368f2b ( 54 143  43)
21 Dark Teal   #329b7a ( 50 155 122)
22 Dark Olive  #778b45 (119 139  69)
23 Dark Blue   #2858a5 ( 40  88 165)
24 Dark Purple #5c3fa3 ( 92  63 163)
25 Dark Maroon #93446b (147  68 107)


Note: color values returned by color meter might be slightly different based on the selected color profile.

Make a custom color and use the RGB Sliders and enter the decimal values above.