If the only difference is that it can't be the exact same color or not bolded, than it's still very hard to recognize the difference.
People know someone is a smod when he has a bright blue username, not when it has a bolded, hexadecimal colored #3333E6 username (asuming smod has marine color.) If the only difference between that and someone who paid 10usd is boldness, I think that there will be alot of people confused.
Even without allowing it to be exactly the same color, it is still very easy to change #3333E6 to #3333E7, nobody sees the difference.
Then, this already existed, and it created confusion. Even though these people had light blue non-bolded usernames, which isn't very close to smod or gm, it still was confusing. There was a reason why you're not able to buy it anymore, even the people that bought these usernames before got their special colors removed.