The green and yellow beans can be blanched 8 hours ahead, covered and refrigerated.
In a large pot of boiling salted water, cook the beans until crisp-tender, about 2 to 3 minutes. Drain well, and then plunge into ice water. Once cold, drain well and pat dry.
In a large bowl, toss the beans, radishes, onions, and basil. Finely grate the lemon zest over the salad. Squeeze the lemon juice over the salad and drizzle with the oil. Sprinkle with salt and pepper and toss to coat.
Transfer the salad to a serving bowl or platter and serve.