Now Microsoft are usually very good with their documentation, but this did initially bring around some frustration as changing background colour simply didn't work.
The article explaining the Set-OMEConfiguration cmdlet is here, and states the following:
The BackgroundColor parameter specifies the background color. Valid values are:
- An HTML hexadecimal (hex triplet) color code value (for example, #FFFFFF is white).
- $null (blank). This is the default value.
However it doesn't work with an HTML colour code as you can see below.
Interestingly utilising Get-Help Set-OMEconfiguration -full didn't shed anymore information on the matter than what I was reading on docs.microsoft.com, even after using Update-Help
Utilising Get-Command Set-OMEConfiguration -Args -backgroundcolor didn't really give me any insights and nor did expanding the parameter set using $a = Get-Command Set-OMEConfiguration
$a.ParameterSet  | select -ExpandedProperty parameters
However this did give me information on what values it was expecting, which could be anything.
In a moment of frustration I just typed a colour in. for example Black, and the change was honoured.
Which frankly is hilarious. As my client wanted a certain blue whilst I was not able to give him the exact blue they wanted I found I could support light blue - although it had to be added as lightblue, 'light blue' was not accepted.
This made me think how many blue options were available to me. It's safe to say you can't have navy blue but royal blue is fully supported.
I've raised this to Microsoft to hopefully get the cmdlet match what's on docs.microsoft.com and give us our HTML colour codes. I am pleased to confirm however $null does indeed work and remove any background colour change you have made :)
Update! 15/06/18 - 48 hours after raising this to Micosoft they have acknowledged the issue and created this article for the list of 140 available colours , as well as updating the original article.