Indesign colors suddenly dull

If you have this happen "suddenly" then I assume you added a transparnt effect like a drop shadow or placed a PNG file. If there is transparency on the page InDesign must show treat the page with the transparency flattener and you will se the result, as mentioned by others in the limitations of your output intent. Onve you are aware that there is the "ideal" or "desired" colour that is not printable you can design with these limitations in mind. If you from the begining activate "proof colours" you will work within the limitations from the start.

If you are intending to design for the web set the Transparency Blend space to RGB. You will keep your bright colours. And colours will blend with RGB logic when there is transparency. (Note that many blend modes are very different in RGB vs CMYK, that is not a bug but just the rules of mathematics)

To see how and why you want to set Transparency Blend Space there are plenty tutorials eg; https://www.printingforless.com/Choosing-a-Transparency-Blend-Space.html

In the color picker, click in one of the RGB values (doesn't matter which one) and you will see it change to "Add RGB Swatch" instead of "Add CMYK Swatch".

Whatever you enter into the Hex Code box will be adjusted to the currently selected color space, so if it says "Add CMYK Swatch" you will get gamut-adjusted colors. Switched to RGB as above and you will be fine.

Also: make sure your document's Transparency Blend Space is set to RGB so you won't get any unwanted color shifts if you do some weird effects.

This is an example of the issue. The already placed items retain and can be that color. But new items cant Adapt it.