For Odoo 12 they're in this  file  web/static/src/scss/secondary_variables.scss in the line 9

// UI custom colors for tags, kanban records' colors, ...)
// Note: the first value is the old BS3 gray-light value
$o-colors: lighten(#000, 46.7%), #F06050, #F4A460, #F7CD1F, #6CC1ED, #814968,
              #EB7E7F, #2C8397, #475577, #D6145F, #30C381, #9365B8;

Try these steps to see the color choices using the UI (version 9, maybe older as well):

1. create the tag text but just leave the default color

2. after the tag is created, click on the tag body.  it will show the colors and then you can pick the one you want.

For data tags we have totaly 11 colar:
Odoo16 /odoo/addons/web/static/src/legacy/xml/base.xml line 1121
# data-color="1" title="Red"
# data-color="2" title="Orange"
# data-color="3" title="Yellow"
# data-color="4" title="Light blue"
# data-color="5" title="Dark purple"
# data-color="6" title="Salmon pink"
# data-color="7" title="Medium blue"
# data-color="8" title="Dark blue"
# data-color="9" title="Fushia"
# data-color="10" title="Green"
# data-color="11" title="Purple"

Duplicate with:
https://www.odoo.com/forum/help-1/list-of-color-codes-30339

https://github.com/odoo/odoo/blob/10.0/addons/web_kanban/static/src/less/kanban_view.less

Here are the relations in Odoo 8.0 from color index to hex value of the color:

0 background-color: #FFFFFF

1 background-color: #CCCCCC

2 background-color: #FFC7C7

3 background-color: #FFF1C7

4 background-color: #E3FFC7

5 background-color: #C7FFD5

6 background-color: #C7FFFF

7 background-color: #C7D5FF

8 background-color: #E3C7FF

9 background-color: #FFC7F1

You can convert the HEX to "real" color via tools like below:

http://www.w3schools.com/colors/colors_picker.asp