I really hate that rationale for 10 point vs. 9 point Colonies.
1. It makes the game more narrow by moving alt - VP strategies and Province strategies from "really hard to pull off" to "impossible to pull off".
2. I tally up most of my games by pairing off one of my duchies with one of my opponents duchies, etc, and finding where the differences are. Using this easy method makes the actual value irrelevant most of the time, Colony could be worth square root of 130 but if we tally it up and you have a Province more than me and I have a Colony more than you do, I can tell who won.
3. "10 looks like a powerful number" and "6 and 9 look the same" are both easily remedied by using different art and formatting on the different cards so that it's easy to tell they are different. There's no reason to blame our Arabic numerals for Rio Grande's sad choice to make all the Treasure cards look as similar as possible and all the Victory cards look as similar as possible as if the game is a test for dyslexia hidden in a board game box. Other games simply don't do that because it's simply neither ergonomic nor pleasant.
Titanic Growth and
Giant Growth are also differently costed cards with different magnitudes of effect, but WotC felt no compulsion whatsoever to confuse players by using identical art.
If this post seems a little overenthusiastic about a single point of change in the value of Colony it's because the sail kinda caught some of the gale from my frustration with trying to teach dominion to new players and every single one having trouble telling treasures apart.