Ok, great.
So, first add a Channel Mixer layer, hit the "Monochrome" checkbox, then enter +100, 0, 0, 0 for the four fields. This will turn the photo black and white. Immediately invert the mask to black.
Then add a Levels layer, and clip it to the Channel Mixer layer. Enter these values:
Red channel 0/0.80/255 and 25/255
Green channel 0/0.80/255 and 60/255
Blue channel 40/0.55.255 and 0/255
Then return to the mask of the CM layer and gently paint over the yellow flowers. The beauty of this method is that you don't need any particular precision - just paint blobs on the areas of yellow. The only places you'll need precision are where the yellow flowers are near the legs and stuff.