KOReader custom status bar color user patch, implements color persitance when changing style fix by @sebdelsol https://gist.github.com/sebdelsol/eba2e492473ac1f9e0ecb003d403b7de

2-change-status-bar-color.lua

local readerFooter = require("apps/reader/modules/readerfooter")
local ProgressWidget = require("ui/widget/progresswidget")
local BD = require("ui/bidi")
local Blitbuffer = require("ffi/blitbuffer")
local Geom = require("ui/geometry")
local Math = require("optmath")
local Screen = require("device").screen

-- Somewhat empirically chosen threshold to switch between the two designs ;o)
local INITIAL_MARKER_HEIGHT_THRESHOLD = Screen:scaleBySize(12)

local PROGRESS_FILL_COLOR = Blitbuffer.colorFromString("#4e77f4") -- Your color here
local PROGRESS_BORDER_COLOR = Blitbuffer.COLOR_BLACK -- Your color here (B/W only)

--[[
COLOR OPTIONS (Lightest to darkest):
    BB.COLOR_WHITE = Color8(0xFF)
    BB.COLOR_GRAY_E = Color8(0xEE)
    BB.COLOR_GRAY_D = Color8(0xDD)
    BB.COLOR_LIGHT_GRAY = Color8(0xCC)
    BB.COLOR_GRAY_B = Color8(0xBB)
    BB.COLOR_GRAY = Color8(0xAA)
    BB.COLOR_GRAY_9 = Color8(0x99) -- was COLOR_WEB_GRAY
    BB.COLOR_DARK_GRAY = Color8(0x88)
    BB.COLOR_GRAY_7 = Color8(0x77)
    BB.COLOR_GRAY_6 = Color8(0x66)
    BB.COLOR_GRAY_5 = Color8(0x55) -- was COLOR_DIM_GRAY
    BB.COLOR_GRAY_4 = Color8(0x44)
    BB.COLOR_GRAY_3 = Color8(0x33)
    BB.COLOR_GRAY_2 = Color8(0x22)
    BB.COLOR_GRAY_1 = Color8(0x11)
    BB.COLOR_BLACK = Color8(0)
    Biltbuffer.colorFromString("#233d68")
    Blitbuffer.colorFromName("red")
    Blitbuffer.colorFromName("orange")
    Blitbuffer.colorFromName("yellow")
    Blitbuffer.colorFromName("green")
    Blitbuffer.colorFromName("olive")
    Blitbuffer.colorFromName("cyan")
    Blitbuffer.colorFromName("blue")
    Blitbuffer.colorFromName("purple")
]]--

local original_init = readerFooter.init

function readerFooter:init()
    original_init(self)

    self.progress_bar.fillcolor = PROGRESS_FILL_COLOR
    self.progress_bar.bordercolor =  PROGRESS_BORDER_COLOR
end

local orig_ProgressWidget_updateStyle = ProgressWidget.updateStyle

function ProgressWidget:updateStyle(thick, height)
    orig_ProgressWidget_updateStyle(self, thick, height)
    self.fillcolor = PROGRESS_FILL_COLOR
    self.bordercolor = PROGRESS_BORDER_COLOR
end

function ProgressWidget:paintTo(bb, x, y)
    local my_size = self:getSize()
    if not self.dimen then
        self.dimen = Geom:new{
            x = x, y = y,
            w = my_size.w,
            h = my_size.h
        }
    else
        self.dimen.x = x
        self.dimen.y = y
    end
    if self.dimen.w == 0 or self.dimen.h == 0 then return end

    local _mirroredUI = BD.mirroredUILayout()
    -- We'll draw every bar element in order, bottom to top.
    local fill_width = my_size.w - 2*(self.margin_h + self.bordersize)
    local fill_y = y + self.margin_v + self.bordersize
    local fill_height = my_size.h - 2*(self.margin_v + self.bordersize)

    if self.radius == 0 then
        -- If we don't have rounded borders, we can start with a simple border colored rectangle.
        bb:paintRect(x, y, my_size.w, my_size.h, self.bordercolor)
        -- And a full background bar inside (i.e., on top) of that.
        bb:paintRect(x + self.margin_h + self.bordersize,
                     fill_y,
                     math.ceil(fill_width),
                     math.ceil(fill_height),
                     self.bgcolor)
    else
        -- Otherwise, we have to start with the background.
        bb:paintRoundedRect(x, y, my_size.w, my_size.h, self.bgcolor, self.radius)
        -- Then the border around that.
        bb:paintBorder(math.floor(x), math.floor(y),
                       my_size.w, my_size.h,
                       self.bordersize, self.bordercolor, self.radius)
    end

    -- Then we can just paint the fill rectangle(s) and tick(s) on top of that.
    -- First the fill bar(s)...
    -- Fill bar for alternate pages (e.g. non-linear flows).
    if self.alt and self.alt[1] ~= nil then
        for i=1, #self.alt do
            local tick_x = fill_width * ((self.alt[i][1] - 1) / self.last)
            local width = fill_width * (self.alt[i][2] / self.last)
            if _mirroredUI then
                tick_x = fill_width - tick_x - width
            end
            tick_x = math.floor(tick_x)
            width = math.ceil(width)

            bb:paintRectRGB32(x + self.margin_h + self.bordersize + tick_x,
                         fill_y,
                         width,
                         math.ceil(fill_height),
                         self.altcolor)
        end
    end

    -- Main fill bar for the specified percentage.
    if self.percentage >= 0 and self.percentage <= 1 then
        local fill_x = x + self.margin_h + self.bordersize
        if self.fill_from_right or (_mirroredUI and not self.fill_from_right) then
            fill_x = fill_x + (fill_width * (1 - self.percentage))
            fill_x = math.floor(fill_x)
        end

        bb:paintRectRGB32(fill_x,
                     fill_y,
                     math.ceil(fill_width * self.percentage),
                     math.ceil(fill_height),
                     self.fillcolor)

        -- Overlay the initial position marker on top of that
        if self.initial_pos_marker and self.initial_percentage >= 0 then
            if self.height <= INITIAL_MARKER_HEIGHT_THRESHOLD then
                self.initial_pos_icon:paintTo(bb, Math.round(fill_x + math.ceil(fill_width * self.initial_percentage) - self.height / 4), y - Math.round(self.height / 6))
            else
                self.initial_pos_icon:paintTo(bb, Math.round(fill_x + math.ceil(fill_width * self.initial_percentage) - self.height / 2), y)
            end
        end
    end

    -- ...then the tick(s).
    if self.ticks and self.last and self.last > 0 then
        for i, tick in ipairs(self.ticks) do
            local tick_x = fill_width * (tick / self.last)
            if _mirroredUI then
                tick_x = fill_width - tick_x
            end
            tick_x = math.floor(tick_x)

            bb:paintRect(x + self.margin_h + self.bordersize + tick_x,
                         fill_y,
                         self.tick_width,
                         math.ceil(fill_height),
                         self.bordercolor)
        end
    end
end

I do not understand how to use this. I put it in my patches folder but I don't see any options for the colour of the progressbar. I'm using the latest stable Koreader version. Is there anything else I need to do?

Sorry this user patch isn't that user friendly, in the first few lines of the code you should be able to write in the color hex code that you want, I never really got round to implementing it into the UI

Ah, I see, I must've missed that. Thanks for the clearup. I downloaded it again, changed the hex code, but now it won't show the progress bar, only the background of the bar with the default grey colour (or the progress bar and the background show the same colour, who knows). Any ideas?

Sorry you've been running into so many issues, I've actually made a much better version of this user patch with proper GUI integration (set the color in the menus) Hope this works better.
https://gist.github.com/IntrovertedMage/6ea38091292310241ba436f930ee0cb4

Nice. I‘ll give it a try. Thanks.

FYI, this one works on my KOReader while your updated one does not, FWIW.