Here are two samples of a technique that I’ve been experimenting with recently, done slightly differently.
In the first case I super-saturated the colors, and then backed off the blue channel a bit because that was a little too much. This give a nice, bright intensity to the leaves. I then copied that background layer, and converted it to a hard, high-contrast B&W in DxO/NIK Silver Efex. Then I used the NIK brush to paint in that B&W effect in from the edges. I did this with 0-hardness, and starting at the corners to enhance the illusion of a triangle. Because the brush was large (2,000 px) and soft with a low flow-rate, the effect is blended in smoothly, though you can see it easily in the corners.
The second shot was similar, but the inverse. Here the base layer was B&W, and I painted back the color from another layer. This was also a large brush tool, but still configured to be very soft.
So, the first I painted in the B&W edging, and the second I painted in the central color.