Color Reference

When defining colors in Roblox, you use

BrickColor.new( <value> )

The value may be a number, OR the name of a color:

BrickColor.new(21)

and

BrickColor.new("Bright red")

both return the color 'Bright red'. (Remember that capitalization must be exact when giving the name of a color!)

Because you can use the names of colors, and the number values corresponding to a color are random at the most, it isn't a very good practice to use the numbers.
However, if you want to, you can use the following color reference:

Color	Value
White	1
Grey	2
Light yellow	3
Brick yellow	5
Light green (Mint)	6
Light reddish violet	9
Pastel Blue	11
Light orange brown	12
Nougat	18
Bright red	21
Med. reddish violet	22
Bright blue	23
Bright yellow	24
Earth orange	25
Black	26
Dark grey	27
Dark green	28
Medium green	29

Please note that this list does not include 'Medium stone grey', as this is the default color for invalid numbers/names (BrickColor.new(-50), BrickColor.new("Not a color name") both return Medium stone grey.)
A list of all of the colors can be found here.
If you would like a more complete reference, you can use the following code:

i = 0
while i < 3000 do
i = i + 1
if tostring(BrickColor.new(i)) ~= "Medium stone grey" then
print(tostring(BrickColor.new(i)) , i )
end
end