If you import your image into the program, next copy it and paste it onto a new layer (or drag it to the create new layer icon), you can now turn this new layer into B/W.
Now you need to mask the flowers, this can be done in various ways and really you should look up some tutorials online to learn various ways of masking, by using a mask you can re-paint things back in if you accidentally paint them out.
Trying to simplify one method, by selecting the mask tool for your B/W layer you will see a new box added on your layers pallette, you can select that and then using a paint brush with either Black or White paint you can paint out parts of your image and re-paint them back in with the opposite colour, ie: paint out with black, and paint back in using white.
Once you have masked out the Flowers you should be able to see the colours showing through from the bottom layer.
A quick and dirty way to get the result is to use the eraser on your B/W layer, simply erase the flowers on that layer to expose the colours from the layer below, use a soft edge eraser for doing this.