I’ve come around two ways for accomplishing this effect.
First option to Create a Color Overlay in Google Slides.
Insert your image, then go to Insert > Shape. I’ve chosen a triangle because I don’t want to apply the overlay to the whole image.
Select your shape and go to Fill Color, select CUSTOM, choose the one you want and adjust its transparency.
And that’s all. The only thing, though, as you can see below, it doesn’t look very sharp. So that’s is why I like best the second option.
Second option to Create a Color Overlay in Google Slides.
Insert your image, select it and copy and paste it (now you have the same image twice) click on Mask Image and select the shape you want, I am going to choose a triangle again.
Note that if you want to apply the overlay to the whole image you can skip the copy, paste and mask step.
With the masked image selected go to Format Options > Recolor and choose one color from the drop-down.
I personally think that this overlay looks much prettier and sharper.
Tip: You may have noticed that the recoloring options are quite limited. This palette is based on the theme. So, if you want to choose a different recolor option you have to follow these steps:
Go to Slide > Edit Master. Click on Colors and Choose a theme color.
Select one accent color and edit it to add the one you want to apply to your overlay. Close the Master and select your picture. Repeat the steps above and when you click recolor, you’ll see the one you’ve added.
As you can see in the video below, red wasn’t a recolor option, so I’ve added it by editing the colors in the master.