Use server.arg results as a function parameter?
server.arg stores get the parameter as a string, so I am not sure what needs to be done. 


The parameter needs to be converted to an uint32_t (and then to a CRGB or CHSV struct). The simplest way is to use the C library function strtoul 

uint32_t color = strtoul(arg, NULL, 16);


This can be improved so that it checks that a number was scanned, etc.

Cheers!