I heard you cant make your tone sound good is this true or nto for screaming and for singing

For singing, to a certain extent what you have heard is correct. The sound of your voice is the sound of your voice, and though you can extend your range and improve your technique, there's not a great deal you can do to change your tone. A good example would be Frank Sinatra, who had quite a narrow range, yet still manages to sell millions of records due to him having a beautiful tone to his voice. Another example on the other end of the scale is (in my opinion) James LaBrie of Dream Theater. He has a great range and technique, and hit plenty of high notes, and hold notes for ages, yet i find his tone to get quite annoying and there isn't much he can do about it. Obviously you can always try to sound like someone else, but it goes without saying that you will get your best results when you sing in your natural tone.
As for screaming, there are numerous different techniques for different sounds, and it's all practise to get the tone you want, which is a lot more variable than your singing tone. Just make sure you're doing it properly.

with singin, everybody wants it to sound perfect, which is why people look for new tones, which unfortunatly u cant do. but screamin is meant to sound fugly (in a good way) so if u hav a tone you dont like, just make ur scream higher or lower, more powerful, less powerful, or wateva. i know that ddnt really help, but i just felt like sayin it...