This can be quite a tricky subject for children to fully understand. If there are 6 yellow cubes and four blue cubes we can say that the ratio of yellow to blue is 6 to 4. This can be simplified to 3 to 2.
The proportion is usually written as a fraction of the whole lot, so the proportion of yellow cubes would be 6 out of 10, or when simplified, 3 out of 5. This can be written as a fraction 3/5.
In the same way, the proportion of blue cubes would be 4 out of 10 or 2 out of 5 (2/5).
A good way to check that the proportions are correct is to add the fractions which should make one.
3/5 + 2/5 = 5/5 or 1.