As I know, there isn't a software for iGO that you can change the colour of maps like a software in TomTom !
You have two options:
you need to download a colour scheme studio software, is free:
and then try to set and change the numbers for example "250,0,0" as you like inside the .zip file of a scheme.
This is a huge work and it needed to know what you change.
Two lovely schemes are here:
[Only registered and activated users can see links. ]
Credit to odytsak
Credit to jgeorg
furthermore you can download this one:
which has a collection by Elchotovolador, 140 best schemes worldwide:
I think that one of the them will be suitable for you!