I have a finite number (around 30,000) of non contiguous integers with values ranging from 1 to, say, 10 billion
I want to spread each of the 30,000 numbers randomly in the number range of 1 to 16 million (I'm trying to give each original number an RGB colour value) so that each original number always get the same value in the range 0 to 16 million.
e.g. 10 is always 323,242 or 123,523,534 is always 242
I think it's something to do with hash functions..but would welcome any help
The simplest is to multiply each integer by $\frac {16,000,000}{10,000,000,000}$. This probably does not satisfy your definition of random. Another approach is to multiply it by some large prime, say with $30$ digits and take the $\bmod 16,000,000$. That might be random enough for you. You could also feed it to some hash algorithm and take the result $\bmod 16,000,000$. That will -feel random but still give you repeatability.