import bpy


material_name = "black_color"
mat = bpy.data.materials[material_name]
colec = bpy.data.collections['Pictograms']

for obj in colec.objects:
    obj.select_set(True)
    bpy.context.view_layer.objects.active = obj
    obj.data.materials[0] = mat
    bpy.ops.object.select_all(action='DESELECT')