Pass Color to a function without using a specific Adafruit_NeoPixel I'm initializing my strips as an array:
// Declare NeoPixel strip array:
Adafruit_NeoPixel strip[] = {
  Adafruit_NeoPixel(LED_COUNT[0], LED_PIN[0], NEO_RGBW + NEO_KHZ800),
  Adafruit_NeoPixel(LED_COUNT[1], LED_PIN[1], NEO_RGBW + NEO_KHZ800),
  Adafruit_NeoPixel(LED_COUNT[2], LED_PIN[2], NEO_RGBW + NEO_KHZ800),
  Adafruit_NeoPixel(LED_COUNT[3], LED_PIN[3], NEO_RGBW + NEO_KHZ800),
  Adafruit_NeoPixel(LED_COUNT[4], LED_PIN[4], NEO_RGBW + NEO_KHZ800)
};

and when I pass Color to a function, I'm arbitrarily using my first strip to create the Color:
  everyother( strip[0].Color( 0, 255, 0), strip[0].Color( 255, 0, 0), 500);

but in the subroutine I use those colors to set pixels to every strip of the array in a loop:
// set every other color to color1 or color2
void everyother( uint32_t color1, uint32_t color2, int wait) {
  for(int k=0; k&lt;NUM_STRIPS; k++) {
    for( int j=0; j&lt;LED_COUNT[k]; j++) {
      if( j%2 == 0) strip[k].setPixelColor(j, color1);
      else strip[k].setPixelColor(j, color2);
    }
  }
  for(int k=0; k&lt;NUM_STRIPS; k++) strip[k].show();  
  delay(wait);
}

Is there another way to define Color that isn't using a real Adafruit_NeoPixel? Yes, it works this way but it feels sloppy.
Looking at the library at https://github.com/adafruit/Adafruit_NeoPixel/blob/master/Adafruit_NeoPixel.h, you can see that the Color method is static (see line 307):
static uint32_t   Color(uint8_t r, uint8_t g, uint8_t b) {
  return ((uint32_t)r &lt;&lt; 16) | ((uint32_t)g &lt;&lt;  8) | b;

This means instead of
strip[0].Color( 0, 255, 0)

You can use:
Adafruit_NeoPixel::Color( 0, 255, 0)

Your call to everyother will be:
everyother(Adafruit_NeoPixel::Color( 0, 255, 0),
           Adafruit_NeoPixel::Color( 255, 0, 0), 
           500);

And since everyother is now not a good name, you can think of a better one, e.g. fillLeds (always start a method name with a verb).
Offtopic:

A subroutine is barely used anymore, I think the last time I heard it, is from the Basic programming language
A function is now used instead
But when a function is inside a class, it's not called a function, but called method.