stock RGBAToHex(r, g, b, a)
{
return (r<<24 | g<<16 | b<<8 | a);
}

stock HexToRGBA(colour, &r, &g, &b, &a)
{
r = (colour >> 24) & 0xFF;
g = (colour >> 16) & 0xFF;
b = (colour >> 8) & 0xFF;
a = colour & 0xFF;
}

stock SetPlayerColorAlpha(playerid, alpha)
{
new r, g, b, a;
HexToRGBA(GetPlayerColor(playerid), r, g, b, a);
SetPlayerColor(playerid, RGBAToHex(r, g, b, alpha));
}