Try this out:
import arcpy, os, traceback, sys
from arcpy import env
env.workspace="in_memory"
env.overwriteOutput = True
try:
def showPyMessage():
arcpy.AddMessage(str(time.ctime()) + " - " + message)
mxd = arcpy.mapping.MapDocument("CURRENT")
color_layer=arcpy.mapping.ListLayers(mxd,"silver.tif")[0]
wholeRaster=arcpy.Raster(color_layer.dataSource)
df = arcpy.mapping.ListDataFrames(mxd)[0]
ext=df.extent
ext='%s %s %s %s' %(ext.XMin, ext.YMin,ext.XMax,ext.YMax)
arcpy.Clip_management (wholeRaster, ext, "victim")
desc = arcpy.GetRasterProperties_management("victim", "MINIMUM")
minV = int(desc.getOutput(0))
desc = arcpy.GetRasterProperties_management("victim", "MAXIMUM")
maxV = int(desc.getOutput(0))
arcpy.AddMessage('%s %s' %(minV,maxV))
intervals=5; valueList=[]
dV=(maxV-minV)/intervals
for i in range(intervals+1):
valueList.append(minV+int(dV*i))
color_layer.symbology.valueField="Value"
color_layer.symbology.classBreakValues =valueList
arcpy.RefreshActiveView()
arcpy.RefreshTOC()
except:
message = "\n*** PYTHON ERRORS *** "; showPyMessage()
message = "Python Traceback Info: " + traceback.format_tb(sys.exc_info()[2])[0]; showPyMessage()
message = "Python Error Info: " + str(sys.exc_type)+ ": " + str(sys.exc_value) + "\n"; showPyMessage()