it's just not how we shade a ball , I did a little edit for you , it will introduce you to ambient lightning , and help you get a more realistic look :

so first dont forget to outline your ball with the darkest color and AA your ball with the background and the inside , and also dont just throw colos like that use some dithering between to enchance the realistic look . and dont forget the shadows follow the curves of the ball , the ball is not flat .
as for ambient lightning i hope this will help :

Okay i dont have much time to fully explain it but if you have any question go ahead ,
hope i helped

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