I've encountered several systems of marked denominations on poker chips...
The first is as follows, and is also what's listed at about.com:
White, $1
Yellow, $2
Red, $5
Blue, $10
Grey, $20
Green, $25
Orange, $50
Black, $100
Pink, $250
Purple, $500
Burgundy, $1000
Light Blue, $2000
Brown, $5000
The second is:
White $1
Red $5
Blue $10
Green $20
Black $100
and the third is
White $1
Red $5
Blue $10
Green $25
Black $100
a fourth is as follows, matching what's at EasyPokerChips.com:
White
1
Pink
2.5
Red
5
Blue
10
Green
25
Black
100
Purple
500
Yellow
1000
Gray
5000
This fourth has gray high, and pink at 2.5...
Wikipedia has a good article on "Gaming Tokens" - it explains the why of $20 chips vs $25 - 5% commissions are common in Pai Gao and Baccarat, and that is aided by use of a $20 chip rather than $25. It's not uncommon in a couple other games, including Roulette and Craps, to have 20:1 payouts, also facilitated by having a $20 chip.