Starting with the October 2020 update, Windows 10 is the default theme that removes bright colors from the taskbar and Start menu. If you want to choose a custom color for the Start menu, there is an easy way to do it through Settings. See how.
First, go to "Settings" by opening the "Start" menu and clicking on icon with the gear on the left. (You can also press Windows + i on your keyboard.)
When "Settings" starts, click "Personalization".
In the "Personalization" settings, click on "Colors" from the sidebar.
In the "Colors" settings, locate the "Choose your color" drop-down menu. In the menu, select "Custom".
After selecting "Custom" from the color menu, two new options will appear below. In the "Choose your default Windows mode" section, select "Dark".
This dark mode is required to color the "Start" menu, but since you have already selected the "Custom" color scheme, you can also use Light mode applications if you wish. So, in the "Choose your default app mode" section, select any of the options ("Light" or "Dark") that you like.
Then scroll down to the "Colors" page and find the "Choose your accent color" section.
If you want the color to automatically match its background image desktop make a selection next to "Automatically pick an accent color from my background". Otherwise, click a color in the grid you want to use for the “Start” menu and the task bar. You can also choose a custom color by doing click in the "Custom color" button below the grid.
After that, locate the section "Show accent color on the following surfaces" and select it frame next to “Start, taskbar, and action center”.
The next time you open the Start menu, you will see that it has changed to Colour you chose.
If you ever want to go back to the classic Windows 10 theme, go to Settings> Personalization> Colors and select "Light" in the "Choose your color" drop-down menu.
Source of information: howtogeek.com