You won't thank me for saying this, but I don't think you have enough data to answer the question.

You don't have any data that tells you how much variation ther is on weight gain under identical conditions. Say you repeated the "plants in green light" experiment a number of times, you would hardly expect to get 12.24g of growth each time.

If you got (and these sre made up numbers)

12.24,

12.28,

12.19,

12.20,

12.25

you could get some idea of the spread of weights that is just due to the natural variation between plants. What you would do would be to calculate a mean and standard deviation for those numbers. (I will let you look up how to do that but the mean is 12.232g and the sd is 0.037g).

If you have a large enough set of data then you can say that 95% of the results will be within 2 times the sd of the average so 95% will be between 12.158g and 12.306g.

Based on that you could say that the 13.47g you got under blue light was odd because it's more than twice the sd away from the average "green" result.

Unfortnately I srtongly suspectr that if you did the experiment on the green lit plants repeatedly you would get a much bigger spread of results and then you couldn't tell if the difference between the green result and the controll result was due to the different lighting, or just part of the natural variation of growth rates.

Did you, by any chance, measure lots of plants and the numbers you gave above are just the total?

If so then you might have enough data to show if there's a real difference or not.

I may get lynched for saying this, but the moral of the story is to talk to a statistician (or learn some statistics) before you do the experiment.