Never mind - I figured out that I should make a hash table and pass that to out-UDChartData instead of my object. (the hint was here: How to create consistent pie chart labels and colors).
New-UDChart -Title "Whats Up-Down" -type pie -AutoRefresh -RefreshInterval 60 -Endpoint {
$UpDown = [ordered]@{Up = ($WhatsUpDown | Where-Object {$_.isOn -eq $true}).count;Down = $($WhatsUpDown | Where-Object {$_.isOn -eq $false}).count} #Works
$UpDown | Out-UDChartData -DataProperty "Values" `
-BackgroundColor @("green", "red") `
-LabelProperty "Keys" `
-HoverBackgroundColor "orange"}
Hope this helps someone else.