[joefromchicago]

I could have sworn there was a "color line" option under League Settings, but I can't find it. Maybe someone with sharper eyes can tell me where it is.

Absent that, for an American-based league, you can go about this one of two ways:

1. In commissioner mode, go into the Explore World page and choose the United States;
2. In the Nation Editor page, change the numbers for all ethnicities other than US (Modern) to zero;
3. Go into the Draft Pool and delete all of the players;
4. Under League Settings/Players, zero out all of the international free agent and scouting options and make sure that the default player origin and amateur draft pool origin settings don't allow for players to come from any other country.

Or, if you want to be more ambitious, you can:

1. Go into the world_default.xml file (located in the game's database folder);
2. Under the United States (nation ID 206) you'll see this:

               <ETHN_PCTS>
               <ETHN_PCT etid="0" pct="2500" />
               <ETHN_PCT etid="1" pct="420" />
               <ETHN_PCT etid="2" pct="2" />
               <ETHN_PCT etid="3" pct="2" />
               <ETHN_PCT etid="4" pct="1" />
               <ETHN_PCT etid="7" pct="1" />
               <ETHN_PCT etid="8" pct="1" />
               <ETHN_PCT etid="9" pct="1" />
               <ETHN_PCT etid="10" pct="1" />
               <ETHN_PCT etid="11" pct="1" />
               <ETHN_PCT etid="12" pct="1" />
               <ETHN_PCT etid="13" pct="1" />
               <ETHN_PCT etid="14" pct="1" />
               <ETHN_PCT etid="15" pct="1" />
               <ETHN_PCT etid="16" pct="1" />
               <ETHN_PCT etid="17" pct="1" />
               <ETHN_PCT etid="18" pct="1" />
               <ETHN_PCT etid="19" pct="1" />
               <ETHN_PCT etid="20" pct="1" />
               <ETHN_PCT etid="21" pct="1" />
               <ETHN_PCT etid="22" pct="1" />
               <ETHN_PCT etid="23" pct="1" />
               <ETHN_PCT etid="24" pct="1" />
               <ETHN_PCT etid="25" pct="1" />
               <ETHN_PCT etid="26" pct="1" />
               <ETHN_PCT etid="27" pct="1" />
               <ETHN_PCT etid="28" pct="1" />
               <ETHN_PCT etid="29" pct="1" />
               <ETHN_PCT etid="30" pct="1" />
               <ETHN_PCT etid="31" pct="1" />
               <ETHN_PCT etid="32" pct="1" />
               <ETHN_PCT etid="34" pct="1" />
               <ETHN_PCT etid="35" pct="1" />
               <ETHN_PCT etid="36" pct="1" />
               <ETHN_PCT etid="37" pct="1" />
               <ETHN_PCT etid="38" pct="1" />
               <ETHN_PCT etid="39" pct="300" />
               </ETHN_PCTS>

3. Change that to this:

               <ETHN_PCTS>
               <ETHN_PCT etid="0" pct="100" />
               </ETHN_PCTS>

Note: although changing the ethnicity to US (Modern) will yield an all-white player pool, those players will have modern names. To get more era-appropriate names, you'll need to download one of the name mods, such as Questdog's historical namesets.