This is Eratosthene's Sieve which gives a method
for computing in a relatively fast manner, what
all the prime numbers are, up to a certain number.
For example, to see which of the first 100 numbers
are prime, start by listing all the numbers using 
a black font.  At the end, those numbers remaining
black are prime.  1 is greyed out as it isn't prime.
Now we color all factorizable numbers in the following
order:
1. color all the even numbers greater than 2 pale green.
2. color all the black multiples of 3 pale blue --except 3.

      [4 has already been colored so go on to 5]
3. color all the black multiples of 5 pale red  --except 5.

      [6 has already been colored so go on to 7]
4. color all the black multiples of 7 orange  --except 7.

      [8, 9 and 10 already colored]
5. STOP because this only needs to be done up to the square root of 100.