There are several possibilities that can cause the light green lawn areas. The Penn State Center for Turfgrass Science department website indicates the following possibilities:
- Poa trivialis - Rough bluegrass is often a contaminant in Kentucky bluegrass seedlots.
- Fusarium Blight - Some Kentucky bluegrass varieties are extremely susceptible to Fusarium blight, especially if nitrogen availability is high. The disease occurs most frequently during hot weather when the plants are under moisture stress.
- Stripe Smut - Although it attacks many turfgrasses, stripe smut is most severe on some varieties of Kentucky bluegrass
In your area the University of Massachusetts Amherst Extension may a good resource (here) to help with your lawn diagnosis.