from tkinter import *

root = Tk()
root.geometry("300x200")


def aktualizuj_kolor(wartosc_z_suwaka):
    v = float(wartosc_z_suwaka)

    if v > 0.4:
        b = "00"
    elif v < 0.38:
        b = "FF"
    else:
        procent = (v - 0.38) / 0.02
        odwrocona_wartosc = int((1 - procent) * 255)
        odwrocona_wartosc = max(0, min(255, odwrocona_wartosc))
        b = f"{odwrocona_wartosc:02X}"
    if v < 0.40 or v > 0.45:
        r = "00"
    else:
        if v <= 0.425:
            procent = (v - 0.40) / 0.025
        else:
            procent = (0.45 - v) / 0.025

        wartosc_r = int(procent * 255)
        wartosc_r = max(0, min(255, wartosc_r))

        r = f"{wartosc_r:02X}"
    g = "00"
    if abs(v - 0.4) < 0.005:
        b = "FF"
        r = "FF"
        g = "FF"

    kolor = f"#{r}{g}{b}"
    canvas.itemconfig(c, fill=kolor)


canvas = Canvas(root, bg="white", highlightthickness=0, width=150, height=150)
canvas.pack(side=LEFT, padx=20)

c = canvas.create_rectangle(20, 20, 120, 120, fill="#FF00FF")

val = Scale(root, from_=0.0, to=1.0, resolution=0.01, command=aktualizuj_kolor)
val.pack(side=RIGHT, padx=20)

root.mainloop()