diff --git a/terracotta/server/rgb.py b/terracotta/server/rgb.py
index a7cf1bc5..5e0cbaff 100644
--- a/terracotta/server/rgb.py
+++ b/terracotta/server/rgb.py
@@ -5,6 +5,7 @@
 
 from typing import Optional, Any, Mapping, Dict, Tuple
 import json
+import re
 
 from marshmallow import Schema, fields, validate, pre_load, ValidationError, EXCLUDE
 from flask import request, send_file, Response
@@ -21,6 +22,36 @@ class RGBQuerySchema(Schema):
     tile_x = fields.Int(required=True, description="x coordinate")
 
 
+def validate_range(data):
+    if isinstance(data, str) and data.startswith("p"):
+        if not re.match("^p\d+$", data):
+            raise ValidationError("Percentile format is `p<digits>`")
+    else:
+        try:
+            float(data)
+        except ValueError:
+            raise ValidationError("Must be a number")
+
+
+
+class StringOrNumber(fields.Field):
+    def _serialize(self, value, attr, obj, **kwargs):
+        if isinstance(value, (str, bytes)):
+            return fields.String()._serialize(value, attr, obj, **kwargs)
+        elif isinstance(value, (int, float)):
+            return fields.Float()._serialize(value, attr, obj, **kwargs)
+        else:
+            raise ValidationError("Must be a string or a number")
+
+    def _deserialize(self, value, attr, data, **kwargs):
+        if isinstance(value, (str, bytes)):
+            return fields.String()._deserialize(value, attr, data, **kwargs)
+        elif isinstance(value, (int, float)):
+            return fields.Float()._deserialize(value, attr, data, **kwargs)
+        else:
+            raise ValidationError("Must be a string or a number")
+
+
 class RGBOptionSchema(Schema):
     class Meta:
         unknown = EXCLUDE
@@ -29,7 +60,7 @@ class Meta:
     g = fields.String(required=True, description="Key value for green band")
     b = fields.String(required=True, description="Key value for blue band")
     r_range = fields.List(
-        fields.String(allow_none=True, validate=validate.Regexp("^p?(\d*\.)?\d+$")),
+        StringOrNumber(allow_none=True, validate=validate_range),
         validate=validate.Length(equal=2),
         example="[0,1]",
         missing=None,
@@ -39,7 +70,7 @@ class Meta:
         ),
     )
     g_range = fields.List(
-        fields.String(allow_none=True, validate=validate.Regexp("^p?(\d*\.)?\d+$")),
+        StringOrNumber(allow_none=True, validate=validate_range),
         validate=validate.Length(equal=2),
         example="[0,1]",
         missing=None,
@@ -49,7 +80,7 @@ class Meta:
         ),
     )
     b_range = fields.List(
-        fields.String(allow_none=True, validate=validate.Regexp("^p?(\d*\.)?\d+$")),
+        StringOrNumber(allow_none=True, validate=validate_range),
         validate=validate.Length(equal=2),
         example="[0,1]",
         missing=None,
diff --git a/tests/server/test_flask_api.py b/tests/server/test_flask_api.py
index f7e5aff8..ac884ea4 100644
--- a/tests/server/test_flask_api.py
+++ b/tests/server/test_flask_api.py
@@ -410,6 +410,8 @@ def test_get_rgb_stretch(client, use_testdb, raster_file_xyz):
 
     for stretch_range in (
         "[0,10000]",
+        "[\"1.0e%2B01\",\"1.0e%2B04\"]",
+        "[\"p2\",\"p98\"]",
         "[0,null]",
         "[null, 10000]",
         "[null,null]",