it is helpful to call it, but arrakis is right, it is not required. And we don't want people to think 'he didn't call green I don't hafta take it' when stabbed in the chest. When me and my wife first started playing she took a red to the field. Hit someone's shield like 4 times and he told her 'you didn't call red so it's not broken'. That is not what we want.
Not to mention battles are so much cooler when every five seconds of it isn't filled with 'red, green, red, red, red, green!'