There is no possible way of predicting what amount of marbles are in the bag. But, to make an accurate and educated guess you should:
Take 10 viewings 6 times (this is when I found the on-screen graph most accurate)and base your predictions on these results. E.g. If there's 19 blue, 19 red, 19 yellow and 3 green, my guess would be 3 blue, 3 red, 3 yellow and 1 green, because, if you divide 19 by 4 (the amount of viewings) you get 4.75, and when you divide 3 by 4 you get 0.75.It is obvious that there is at least 1 green within the bag, so you should keep this in mind. I didn't put the blue, red or yellow as 4, since 3x4 is 12, and out of my marble prediction allowance of 10. The next lowest whole number was 3, which would be appropriate as 3x3 + 1 = 10. I got this as a result in one of the games, made my guess and I scored 100 points.
If you use this strategy throughout the game, you will achieve 1000 points over several viewings. I did whilst using this concept.