A way of doing that would be by using the following table:

Number	Colour
0	00.00.00
255	00.00.FF
510	00.FF.FF
765	00.FF.00
1020	FF.FF.00
1275	FF.FF.FF
1530	FF.00.FF
1785	FF.00.00
2040	00.00.00

Intermediate value are calculated by linear interpolation. This gives you 2040 different colours; nearby values are mapped to nearby numbers; and values far apart will have quite different values (with the caveat that the table above is cyclic). It shouldn't be too hard to turn this table into code.

If you just have 1000 numbers, multiply the numbers by 2 before consulting the above table, or just use a subset of the colours.

If you have a lot of numbers, much more numbers than there are different values, it probably pays to make a lookup table.