Apple offers a decent set of tools to edit images. Most of use highlighter tool to sensor sensitive data from pictures or hide private data. I have seen people using the highlighter tools to hide their credit card information amongst many other things. A Redditor has explained why using Apple’s highlighter tool to censor sensitive data is a bad idea.
The Redditor has explained how the default opacity of the tool is a problem. If the value of opacity is less then 100 then the tool will not be able to successfully censor data. In other words, the censored picture is semi-transparent and one can see through the marking. However, one can still photoshop to access the censored information even when the opacity is set almost to the maximum.
The OP used a highlighter from the default iOS toolset to color text. Now it seems like the text is censored and not readable. However, he opens the image in default iOS photo editor and toggles settings like exposure, highlights and more. Once the settings are changed, the image comes to life and you can easily see the censored information. As you have seen above, using highlighter on iOS to conceal information is not really a bright idea. Even if you do, make sure that you set the opacity to 100. You can also consider using third-party apps that let you conceal or even blur information.
The iOS 13 comes with an updated photo editor that is much simpler, with an Instagram style editing interface. You can swipe between the tools at the top, and then use the slider to change the intensity You’ll find the basic editing features like Brightness, Contrast and so on. Apple has also added new tools like Highlights, Vibrance and more.