ok this are what red turnips are:
you buy them from joan on sunday for 1000 bells, then you plant them in the ground. Water them every day for a week (if you forget it dies) and then you sell it to nook for 15k which isnt that much
now white turnips are bought from joan for like around 100 bells for 10
you wait for someone to have a high turnip price and then you sell your turnips
but sometimes no1 has a high price and you waste your money because white ones die after a week