have duplicated the background layer (click Layer/Duplicate Layer)
and set the general effect mode for both layers to Multiply.
This way, the raspberry got darker and more saturated colors. Of
course, you can select any effect you like in this step, according
to your photo and concept. This first step is actually unimportant
from the aspect of layer masking. The point is to have at least
two layers that differ in, say, their general effect.
the example, our aim is to keep the changes of colors and lightness
on the raspberry while preserving the original state on the rest
of the picture.
the indicated icon at the bottom of the Layers palette to
create a layer mask on the upper layer. This will be indicated by
an empty white rectangle with a narrow light border, showing that
a layer mask is selected. Make sure it stays so.
put, a layer mask allows you to punch through the layers so that
the layer beneath shows through in certain areas. The most appropriate
tool for this aim is Eraser (press E to activate it).
Choose a mid-sized soft brush from the options at the top. Start
painting over the areas where you want to restore the original state.
In this example, these are the leaves, except the raspberry and
the dark background. As the layer mask is active, you will see the
changes on it. In the layer mask indication window, a black color
indicates the punched-through areas. In these parts, the layer under
the topmost one will show through, while white areas still display
the upper layer.
you have messed up something, and want to paint an area back, change
to the Brush tool and choose a brush of similar size and
properties to restore the upper layer, shown once again in white.
you're ready, click Layer/Flatten Image to merge the layers.
Here's the result.