Composite video only needs the 75 ohm load resistor if the input to the TV is high impedance. Almost all TVs have a 75 ohm input impedance already so the load resistor isn't needed - the TV is the load 
According to the data sheet there is a slight output resistance on the digital outs, you may find 82 ohm resistors work better than 68. Try it, it will either look better or worse. I did write a sketch to test how monotonic the DAC is - put a 75 ohm load resistor in place for this (doesn't have to be exactly 75 ohms) and connect the output to analog in 0:
// array runs lsb to msb, you can add/remove
// values according to the number of resistors
// in your DAC
char pins[]={34,35,36,37,38,39,40,41};
void writeval(int v){
 for(int i=0;i<sizeof(pins);i++){
  digitalWrite(pins[i],v&1);
  v>>=1;
 }
}
void setup() {
 Serial.begin(115200);
 analogReadResolution(12);
 for(int i=0;i<sizeof(pins);i++)
  pinMode(pins[i],OUTPUT);
}
void loop() {
 int l=0;
 for(int i=0;i<(1<<(sizeof(pins)));i++){
  writeval(i);
  delay(10);
  int m=analogRead(0);
  printf("%d : %d d=%d\n",i,m,m-l);
  if(m<l)printf("!!!!!!ERROR!!!!!!!!!! non-monotonic\n");
  l=m;
 }
 delay(2000);
}
Because of the number of technical compromises that have to be made to get a colour composite signal at all, there will always be a lot of colour fringe. The output signal is way off spec in a number of areas. I aimed low with that project, the goal was VHS quality rather than broadcast quality 