People who are color blind will have problems interpreting visuals that are highly dependent on color.
The most common type of color blindness makes it hard to tell the difference between red and green. Another type makes it hard to tell the difference between blue and yellow.
Although possible, it is rare to find people who do see color at all. However, using a black-and-white scale is a good starting point (and an easy one) to test whether your graphics have the desired impact on people who have some form of colorblindness.
Review an Image
In PowerPoint, right-click on the picture and go to the format picture tab. Then click on re-color and select the black and white panel.
Review the Slideshow
To review the entire PowerPoint presentation, click the View tab. The default is "Color." Click on with "Grayscale" or "Black and White" tab to remove the color from all slides. The "Color" tab will remain visible to convert easily the slides back into their original state.
Moving Forward
Of course, you could also go old school and simply print the image or the slides in black and white.
The important point is to make an intentional effort to understand how your visuals are seen by people with some form of color blindness. Communicate with FINESSE!