A lot of purple plants which have been created (cultivars) will revert to the green of the original variety. This is very common with basil, especially. Not much you can do about it except remove the green leaves and hope the replacement batch will be purple.

I purchased seeds this year that where exactly that...I'll have to look to see what they called it...but it green just like the pic...green and purple...and it was a very hearty plant, tasty too.

It never occured to me that Purple Basil is a hybrid... :) Huh! Well, that makes sense. As I said, the seeds had not sprouted for a while.

Thank you! It was still, although a tad skinny, a good plant. ;)

2 Varieties you may like to try are Purple Ruffles and Osmin. Both are solid purple including the stems.

Perhaps you planted Osmin Purple Basil seed. It is one of the earlier purple cultivars that was developed and is not always reliably purple. A certain percentage of seeds still will yield a few green and also what I think are spectacularly lovely variegated plants that look as if someone went bonkers with a paintbrush and spattered green leaves with purple splotches or purple leaves with green splotches! I think most nurseries cull out the "sports" of this kind of basil and pot on only the true purple ones as I never have seen the variegated plant for sale commercially.
The small nursery I used to work at culled the variegated Osmin seedlings, anyway--and allowed me to take all the discarded ones I wanted! The odd ones smell & taste just as good as any standard basil, with the added benefit of being an unusual, beautiful, edible garnish. I plant it each year because it is so pretty and it always draws lots of comments from visitors to my garden. CK

Here is a link that might be useful: Osmin purple basil