After some searching around it seems like you are correct that there is not built-in hook to override the pre-prompt [computername]: tag.
Luckily, I have a hacky workaround which could work for you!
To get color, we can just use Write-Host. Write-Host output from the prompt function will be fully left-justified, which is what we want. Unfortunately, the default [computername]: tag is inserted directly afterward. That results in the computer name being duplicated in the prompt, once with color and once without.
We get around this by returning a string containing backspace characters, so the un-colored [computername]: will be overwritten. This is the normal prompt string, typically the current path.
Finally, in case the normal prompt string is short and does not fully overwrite the un-colored [computername]: tag, we need to do some final cleanup by adding dummy space characters. That might push out the caret, though, so we need to add more backspaces to return the caret to the corrent position.
All-up, use this on your remote machine:
# put your logic here for getting prompt color by machine name
function GetMachineColor($computerName)
{
[ConsoleColor]::Green
}
function GetComputerName
{
# if you want FQDN
$ipProperties = [System.Net.NetworkInformation.IPGlobalProperties]::GetIPGlobalProperties()
"{0}.{1}" -f $ipProperties.HostName, $ipProperties.DomainName
# if you want host name only
# $env:computername
}
function prompt
{
$cn = GetComputerName
# write computer name with color
Write-Host "[${cn}]: " -Fore (GetMachineColor $cn) -NoNew
# generate regular prompt you would be showing
$defaultPrompt = "PS $($executionContext.SessionState.Path.CurrentLocation)$('>' * ($nestedPromptLevel + 1)) "
# generate backspaces to cover [computername]: pre-prompt printed by powershell
$backspaces = "`b" * ($cn.Length + 4)
# compute how much extra, if any, needs to be cleaned up at the end
$remainingChars = [Math]::Max(($cn.Length + 4) - $defaultPrompt.Length, 0)
$tail = (" " * $remainingChars) + ("`b" * $remainingChars)
"${backspaces}${defaultPrompt}${tail}"
}
