SharePoint online json

I would like to add custom code to my sharepoint list to change colour and add an icon. i found the below code but i don’t know how to customise the colours? 

{
"$schema":"https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType":"div",
"attributes": {
"class":"=if(@currentField == 'Done', 'sp-field-severity--good', if(@currentField == 'In progress', 'sp-field-severity--low', if(@currentField == 'In review', 'sp-field-severity--warning', if(@currentField == 'Has issues', 'sp-field-severity--severeWarning', 'sp-field-severity--blocked')))) + ' ms-fontColor-neutralSecondary'" },
"children": [
    {
"elmType":"span",
"style": {
"display":"inline-block",
"padding":"0 4px" },
"attributes": {
"iconName":"=if(@currentField == 'Done', 'CheckMark', if(@currentField == 'In progress', 'Forward', if(@currentField == 'In review', 'Error', if(@currentField == 'Has issues', 'Warning', 'ErrorBadge'))))" }
    },
    {
"elmType":"span",
"txtContent":"@currentField" }
  ]
}