import panel as pn
pn.extension()

import random

rcolor = lambda: "#%06x" % random.randint(0, 0xFFFFFF)

box = pn.GridBox(*[pn.pane.HTML(background=rcolor(), width=50, height=50) for i in range(24)], ncols=6)
box

color = pn.pane.HTML(background=rcolor(), width=50, height=50)
box[5] = color

box.ncols = 4

box