With just the addition of a Gradient Map adjustment layer, you can add all kinds of color effects to your image, as I'll show you here.
key concepts:
adjustment layers
Here's the original image.
Step One.
Click on the Create New Adjustment Layer icon at the bottom of the Layers palette, and from the pop-up menu, choose Gradient Map. In the Gradient Map dialog, click on the down-facing triangle next to the gradient thumbnail to get the Gradient Picker. If black and white are not your current Foreground and Background colors, choose the third (Black, White) gradient from the left in the top row, otherwise you can use the Foreground to Background gradient in the top left.
This will add a black-and-white effect to your color photo, as you can see here.
Step Two.
If you clicked OK, double-click on the adjustment layer's thumbnail to reopen the Gradient Map dialog, then click once on the gradient thumbnail itself (not the triangle) to open the Gradient Editor.
Step Three.
In the bottom half of the Gradient Editor, double-click on the first (far left) black color stop to choose the main color you want to use. In the resulting Color Picker, after choosing your color, highlight the hexadecimal color number (underneath the Blue value) and press Command-C (PC: Control-C) to Copy it.
Next, double-click on the last (far right) white color stop and in the Color Picker, press Command-V (PC: Control-V) to Paste the hexadecimal color number in its field (this is a simple way to ensure both color stops are the same color). Then click below the color bar, roughly in the middle, to add another color stop. Double-click on the new color stop and change the color to white in the Color Picker. As you do these operations, you'll see the effects you're having on the image.
Step Four.
Continue to experiment with different colors for the color stops, and click-and-drag the color midpoints (the small diamond shapes between the color stops) to vary the distance of the blend between colors.
Variations
Here I used the Gradient tool (G) to paint with a Radial gradient in black on the layer mask so the original flower shows through in the center.
Variation 1: Radial gradient
In this case, I Control-clicked (PC: Right-clicked) on the adjustment layer and chose Blending Options. Then I held down the Option key (PC: Alt key) and split the black and white This Layer Blend If sliders, dragging until I liked the results. (For more on this, see Blending Options in the Key Concepts.)
Variation 2: Split Blend If sliders
This example has the Blending Options as in the previous variation, and a Radial gradient as in the first variation.
Variation 3: Combined Variations 1 and 2
Here I copied the contents of the Red channel and then pressed-and-held the Option key (PC: Alt key) while I clicked on the layer mask. This allowed me to view the layer mask so that I could paste the copied channel and create a "photographic" mask.
Next, double-click on the last (far right) white color stop and in the Color Picker, press Command-V (PC: Control-V) to Paste the hexadecimal color number in its field (this is a simple way to ensure both color stops are the same color). Then click below the color bar, roughly in the middle, to add another color stop. Double-click on the new color stop and change the color to white in the Color Picker. As you do these operations, you'll see the effects you're having on the image.
