A função SelectObject(hdc, GetStockObject(WHITE_PEN) define a cor da linha no contexto do dispositivo - Post reescrito em 06/10/2023

#include <windows.h>
#include "resource.h"

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hI, HINSTANCE hPI, PSTR szCL, int iCS) {
    static TCHAR szAppName[] = TEXT("EndJoin");
    HWND hwnd;
    MSG msg;
    WNDCLASS wndclass;
    
    wndclass.style = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc = WndProc ;
    wndclass.cbClsExtra  = 0;
    wndclass.cbWndExtra  = 0;
    wndclass.hInstance   = hI;
    wndclass.hIcon   = LoadIcon(hI, IDI_ICON);
    wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName  = NULL;
    wndclass.lpszClassName = szAppName;

    if(!RegisterClass(&wndclass)) {
        MessageBox(
            NULL,
            TEXT("This program requires Windows NT!"),
            szAppName,
            MB_ICONERROR
        );
        return 0;
    }
    hwnd = CreateWindow(
        szAppName, TEXT("Programação para Windows"),
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT,
        CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hI, NULL
    );
    ShowWindow(hwnd, iCS);
    UpdateWindow(hwnd);
    
    while(GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage (&msg);
    }
    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wP, LPARAM lP) {
    static int iEnd[] = {
        PS_ENDCAP_ROUND, PS_ENDCAP_SQUARE, PS_ENDCAP_FLAT
    };
    static int iJoin[]= {
        PS_JOIN_ROUND, PS_JOIN_BEVEL, PS_JOIN_MITER
    };
    static int cxClient, cyClient;
    HDC hdc;
    int i;
    LOGBRUSH lb;
    PAINTSTRUCT ps;
    switch(msg) {
        case WM_SIZE:
            cxClient = LOWORD(lP);
            cyClient = HIWORD(lP);
            return 0;
        case WM_PAINT:
            hdc = BeginPaint(hwnd, &ps);
            SetMapMode(hdc, MM_ANISOTROPIC);
            SetWindowExtEx(hdc, 100, 100, NULL);
            SetViewportExtEx(hdc, cxClient, cyClient, NULL);
            lb.lbStyle = BS_SOLID;
            lb.lbColor = RGB(0, 82, 0);
            lb.lbHatch = 0;

            for(i = 0; i < 3; i++) {
                SelectObject(
                    hdc,
                    ExtCreatePen(
                        PS_SOLID | PS_GEOMETRIC |
                        iEnd [i] | iJoin [i], 8,
                        &lb, 0, NULL)
                );
                BeginPath(hdc);
                MoveToEx(hdc, 10 + 30 * i, 25, NULL);
                LineTo(hdc, 20 + 30 * i, 75);
                LineTo(hdc, 30 + 30 * i, 25);
                EndPath(hdc);
                StrokePath(hdc);
                DeleteObject(
                    SelectObject(hdc, GetStockObject(WHITE_PEN))
                );
                MoveToEx(hdc, 10 + 30 * i, 25, NULL);
                LineTo(hdc, 20 + 30 * i, 75);
                LineTo(hdc, 30 + 30 * i, 25);
            }
            EndPaint(hwnd, &ps);
            return 0;
        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }
    return DefWindowProc(hwnd, msg, wP, lP);
}

/********************Arquivo: resource.h********************/

#define IDI_ICON " "

/********************Arquivo: resource.rc********************/

#include "afxres.h"
#include "resource.h"

IDI_ICON ICON "project1.ico"