Try JSON below and modify as needed. In your condition, you have not mentioned the color for the value outside the condition, that is, else condition. I used "gray", but you can change it to your liking.
You can visit MS site for more JSON formatting examples.
{
"$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
"elmType": "div",
"txtContent": "@currentField",
"style": {
"background-color": "=if(@currentField >=1 && @currentField <=3, 'Green', if(@currentField >=4 && @currentField <=6, 'Yellow', if(@currentField >=8 && @currentField <=12, 'Orange', if(@currentField >=15 && @currentField <=25, 'Red', 'gray'))))",
"color": "black",
"font-size": "2.0em",
"justify-content": "center"
}
}
