win32 - how testing pixels of the WS_TRANSPARENT control?

[cambalinho]

the control have the WS_TRANSPARENT style. and:

case WM_ERASEBKGND:
            {
                return TRUE;
            }
            break;
            case WM_CTLCOLORSTATIC:
            {
                return (LRESULT)GetStockObject(NULL_BRUSH);
            }
            break;
            case WM_DRAWITEM:
            {
                DRAWITEMSTRUCT *test=(DRAWITEMSTRUCT*) lParam;
                image imglabel(test->rcItem.right-test->rcItem.left,test->rcItem.bottom-test->rcItem.top);
                if(inst->blnTransparent!=true)
                    FillRect(test->hDC,&test->rcItem, CreateSolidBrush(inst->clrBackColor));
                else
                    FillRect(test->hDC,&test->rcItem,(HBRUSH) GetStockObject(NULL_BRUSH));
                FillRect(imglabel,&test->rcItem,CreateSolidBrush(inst->clrBackColor));
                if(inst->imgtest.haveimage())
                    DrawHICONtoHDC(imglabel, inst->imgtest,1,1);
                SetBkMode(imglabel,TRANSPARENT);
                char *text=(char*)inst->strCaption.c_str();
                SetTextColor(imglabel,inst->clrTextColor );
                DrawTextEx(imglabel,text,-1,&test->rcItem,DT_LEFT,NULL);
                if(inst->blnBorder==true)
                    DrawEdge(imglabel, &test->rcItem,BDR_SUNKENINNER | BDR_RAISEDOUTER,BF_RECT);
                imglabel.Backcolor=inst->clrBackColor;
                TransparentBlt(test->hDC,0,0,test->rcItem.right,test->rcItem.bottom,imglabel,0,0,test->rcItem.right,test->rcItem.bottom, inst->clrBackColor);
                return TRUE;
            }
            break;

these code is what show us the label transparent.
now with click i did these:

case BN_CLICKED:
                    {
                        HDC Hdc=GetDC(inst->hwnd);
                        POINT curpos;
                        GetCursorPos(&curpos);
                        ScreenToClient(inst->hwnd,&curpos);
                        if(GetPixel(Hdc,curpos.x ,curpos.x)!=CLR_INVALID && inst->blnTransparent==true)
                            inst->MouseClick();
                        else if(inst->blnTransparent==false)
                            inst->MouseClick();
                        ReleaseDC(inst->hwnd,Hdc);
                    }
                    break;

(my label is the win32 button class)
when the control is transparent the pixel(backcolor) isn't showed. but how can i test that pixels?
that code works, but it's instable(works sometimes) :(