Well if you are trying to get rid of the if statement and replace a color based on rgb i guess maybe something like so.
// 1 or 0 only.
float lerpFlag = saturate( sign(abs( (OldColor.r + OldColor.g + OldColor.b) - (color.r + color.g + color.b))) );
return lerp(NewColor, color, lerpFlag);
Though i dunno what you are trying to say with the color there? If its just supposed to be more then zero then execute? Then just get the sign of it and multiply it with the lerp flag. * sign(color.a);