How to set color object using python and Blender 3.0 individually using a template object

Question

I have tried all the previous solutions but I can not change the colour of objects individually depending on a condition, the following code will produce 4 red cube instead of 2 red and 2 green:

template_object = bpy.data.objects.get('Cube')
matg = bpy.data.materials.new("Green")
matg.diffuse_color = (0,1,0,0.8)
matr = bpy.data.materials.new("Red")
matr.diffuse_color = (1,0,0,0.8)

manlist = [
{"color" :"red"},
{"color": "green"},
{"color": "green"},
{"color": "red"},
]
for man in manlist:
    ob = template_object.copy()
    if man['color'] == 'red':
        ob.active_material = matr
        print('red')
    if man['color'] == 'green':
        ob.active_material = matg
        print('green')
    bpy.context.collection.objects.link(ob)

Answer 1

As Marty Fouts points out in the comments, your material slot is linked to a mesh, and all objects are linked to the same mesh. Probably the best way to resolve this is to link the material slot to the object:

import bpy

template_object = bpy.data.objects.get('Cube')
template_object.material_slots[0].link = 'OBJECT'
matg = bpy.data.materials.new("Green")
matg.diffuse_color = (0,1,0,0.8)
matr = bpy.data.materials.new("Red")
matr.diffuse_color = (1,0,0,0.8)

manlist = [
{"color" :"red"},
{"color": "green"},
{"color": "green"},
{"color": "red"},
]
for man in manlist:
    ob = template_object.copy()
    if man['color'] == 'red':
        ob.active_material = matr
        print('red')
    if man['color'] == 'green':
        ob.active_material = matg
        print('green')
    bpy.context.collection.objects.link(ob)

Answer 2

You have two basic problems.

First, diffuse_color is only the display color used for an object in Solid Viewport shading. You need to actually create a material if you want the color in rendering. Here's green:

matg = bpy.data.materials.new("Green")
matg.use_nodes = True
tree = matg.node_tree
nodes = tree.nodes
bsdf = nodes["Principled BSDF"]
bsdf.inputs["Base Color"].default_value = (0, 1, 0, 0.8)
matg.diffuse_color = (0, 1, 0, 0.8)

(Notice that while I was at it, I gave you a green color instead of a blue one for "Green")

Red would similar:

matr = bpy.data.materials.new("Red")
matg.use_nodes = True
tree = matg.node_tree
nodes = tree.nodes
bsdf = nodes["Principled BSDF"]
bsdf.inputs["Base Color"].default_value = (1, 0, 0, 0.8)
matg.diffuse_color = (1, 0, 0, 0.8)

Second, your assignment loop is OK, except that the mesh copy() command is linking the mesh of the template object to each of the four new objects, so you only have one mesh. You're setting the color of that mesh by the last entry in your loop and that happens to be red.

There are several ways to duplicate the mesh without linking. It seems like using the duplicate operator would work, but I'll leave it to you to select an appropriate one. But here's how to do it with duplicate

for man in manlist:
    bpy.ops.object.duplicate()
    ob = bpy.context.active_object
    if man['color'] == 'red':
        ob.active_material = matr
        print('red')
    if man['color'] == 'green':
        ob.active_material = matg
        print('green')
    bpy.context.collection.objects.link(ob)

Answer 3

Here is how I create color for an object using Goo package to create a cell object:

#====================== Colors =========================
# Green
matg = bpy.data.materials.new("Green")
matg.diffuse_color = (0,0.1,0,0.8)
# Red
matr = bpy.data.materials.new("red")
matr.diffuse_color = (0.1,0,0,0.8)

#================== Cell A Collection ==================
# Create a collection for cell A
cA_collection = bpy.data.collections.new("A_Cells")
# link the collection to the scene for visualization 
bpy.context.scene.collection.children.link(cA_collection)
# Define cell A1
cA1 = goo.Cell("cell_A1", loc = (2,2,0))
# Make a Blender mesh object for cell
goo.make_cell(cA1)
# The created cell is the active object
obj = bpy.context.active_object
# Set the object color
obj.active_material = matg
# Remove object from all collections not used in a scene 
object inside the collection
bpy.ops.collection.objects_remove_all()
# Add the active cell to our specific collection 
bpy.data.collections['A_Cells'].objects.link(obj)