Hope you are doing well, and thanks for reaching out and for including the screenshot.
As described, instead of the full color palette, the highlight color picker sometimes shows a reduced view (an opacity slider or just a few basic shades) rather than the full range of colors. A couple of things to try:
In the color picker, tap directly on the circular swatch showing the currently selected color, this sometimes opens up the full palette of colors rather than the limited view you're seeing
Sign out of the app completely, restart your device, then sign back in with the same Adobe ID
Check for a pending app update available on the App/Play Store and reboot the device once, and see if that works.
A couple of things that would help confirm this further: what device and OS are you using, and what version of the app do you have installed? Also, does this happen with every PDF you try to highlight, or only certain ones?
Let us know if tapping the color swatch brings back the full palette, and your device/version details, that'll help narrow it down further.
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