To edit the color of the yellow triangle shape, you can double click the precomp in the timeline:

1. Select the bottom yellow layer
2. Go to the Effect & Presets panel
3. Type fill
4. Double click in the effect

This will add that effect to what ever you have chosen. As you can see you get a effects panel that pops up next to your project panel. The default color for this effect is red. Adjust your color by selecting the color box and selecting a new color:

Now go back to your main comp. You will see your background is a different color. Spend some times adjusting the colors of your shapes and background. Use the same fill effect on anything you want to change the color of. You want your text for your lower thirds to stand out from the background of these shapes.

In After Effects you have access to many image editing tools that come with Photoshop. Were going to use one called curves. Select your photo background layer. Find the effect called curves in the Effects & Preset panel. Click on this effect to add it to your photo layer. Look at the effects control panel next to your project window. Adjust your curve effects using the bottom button called auto:

Next we will add a photo filter effect to our image. Find this in the effect panel by typing photo filter. Double click and add this effect to your layer. This effect will give your image an overall color effect. Choose a nice cooling filter (80).
image photo filter