I'm not a colour expert so basically just did it all randomly. Back in the day we had a Hannah who did all this.
First post edited and have taken on board your suggestions and changed every title, hope it's a bit better
You should change this rank : 2500 posts is FJ Addict
Nobody wants to have "addict" or "addiction" in their rank status. There is a site that I am a member on and the highest rank is "Insert Username is Site Name Addicted" (Which I am unfortunately a part of). Many people are offended with the rank name.
You can remove that and add FJ Pro or you can take my ranking suggestion :-
1 post is called FJ Newbie = 1 default star & Grey colour username
50 posts is FJ Beginner = a light green star & colour
125 posts is FJ Junior = 1 seafoam star & green colour
250 posts is FJ Regular = 2 purple stars @ purple colour
500 posts is FJ Senior = 2 Teal stars & brown colour
1000 posts is Loves FJ = 3 light blue stars & ligh blue colur
2500 posts is Adores FJ = 3 blue stars & blue colour
5000 posts is FJ Pro = 4 orange stars & orange colour
7500 posts is FJ Star = 4 dark blue stars & dark blue colourr
10,000 posts is FJ Legend = 5 red stars & red colour
Now about the colors.
First you must consider that the colors you choose must be properly visible on both dark and light themes. To make the workload small, you could ignore the dark themes for now. If we take the light themes, then you should use darker shades of the colors. If you use lighter shades on light themes then they won't be properly visible. This also applies to dark themes.
Now, we have 10 Ranks and 3 staff groups (I assume this forum only has mod, Gmod and admin ?). That's a total of 13 Colors that is needed.
Now, let's make a list of colors. I will give the colors codes which look very good on a theme with a pure white background (You need a good display to see how they look) which I use on my site. I obtained the color codes after a lot of googling and observation. It would be a wise to keep a record of your current color codes just in case people do not like them and ask a rollback.
1) Gold : #ce960b
2) Silver : #6b6363 (You can make this shade a bit lighter)
3) Brown
4) Red : #ff0000
5) Orange : #ff7b00
6) Green : #008000
7) Indigo/Dark Blue : #0000cd
8) Cyan : #00b1b1
9) Purple : #68228b
10) Pink : Your current color is the best one
11) Black : #000000
Other Extra colors which are variants of the above are :-
12) Sea Green : #098f7a
13) Dark Lime
14) Maroon
15) Light Green :
Here is how I would make the color distribution :-
Admin - Red
GMod - Green
Mod - Indigo/Dark Blue
Ranks
Level 1 - Black
Level 2 - Purple
Level 3 - Sea Green
Level 4 - Cyan
Level 5 - Orange
Level 6 - Gold
Level 7 - Maroon
We can brainstorm and decide which colors to fill in the remaining 3 groups with.