[Kwarde]

Its impossible indeed (as far as I know) to change it ingame. If you wanna change such thing, make a global variable. eg. the COLOR_CUSTOM:

pawn Код:

#include <a_samp>
#include <zcmd>

new COLOR_CUSTOM = 0xFF0000AA; //Red

CMD:current_color(playerid, params[])
    return SendClientMessage(playerid, COLOR_CUSTOM, "This is the current custom color!");

CMD:set_color(playerid, params[])
    return isnull(params) ? SendClientMessage(playerid, 0xFFFFFFAA, "USAGE: /set_color [color]") : CUSTOM_COLOR = params[0]; //Not sure if that'll work :P

Something similar to that.
The only way to change a DEFINE is, as said above with #undef and again #define

pawn Код:

#define COLOR_CUSTOM 0xFF0000AA //Red
#undef COLOR_CUSTOM
#define COLOR_CUSTOM 0x00FF00AA //Green