Wow finnie explained that great, if you know whether your budgie is split green and blue or just green you can use punnet squares, this is a very simple method as the are lots of genes affecting shades and whether it is clearwing, cinnamonwing and others. But here is a simple thing u can do to predict the offspring if you know whether or not they are split for blue.
If G is green and b is blue
And for example mother and father are split for blue i.e. Gb then it would look like this
G b
G GG Gb
b Gb bb
There for 25% will be GG or green budgies not split for blue
There will be 50% Gb, or Green budgies split for blue
And 25% bb, blue budgies.
Hope this method will help u to be able to predict future offspring if you know some of their characteristics.