The leaves on my pink Alice Dupont Mandevilla plant are turning yellow.
It sounds as if your plant may be watered too much. I would suggest you remove the yellow leaves and then check the soil around the plant every two or three days to see if it is too moist or too dry. If the soil still is moist when you insert one fingertip into it, your plant will not need water for three or four days. Even then, water it just enough to moisten the soil and see if the leaves stop turning yellow. One thing about Mandevilla vines is that they should be trimmed and bit and brought indoors for the winter because they cannot withstand temperatures below 60 degrees. With good care and enough indoor light, your plant should survive indoors and be ready to put outside next spring when night-time temperatures are above 60 degrees.