the colours in start menu can take the dominant colour on desktop and use it as part of theme.
right click desktop and choose Personalise
look on colours tab and scroll down to under Choose accent color
Is there a tick next to Automatically pick an accent colour from my background
you can unclick that and choose a color from colour box underneath
or you can scroll to bottom and choose where the colours are used
if you untick the choices at the bottom, the default for theme will show, this can either be grey or white, depending what you have chosen in top box on tab that says Choose your colour
These choices also affect colours shown at start/shutdown