24783

Step 1: Take 5 veiwings and record the results for each mables' colour.
Step 2: Find the average for each marbles' colour - if the average for a colour is deciml round to one unit.
Step 3: One whole number of a colour equals one marble of that colour.
Step 4: Make your guess. This solution should get 100 points.
Step 5: Repeat another 9 times.

This solution works because each time you are subtracting 50 but adding 100(therefore adding 50). 500 + (50*10) = 1000.