How to fill a region of the channel of a trackbar without changing the appearance of the slider

I need to fill some regions of the channel of a trackbar with different colors, but if the slider (most of its area) is located in such a region, the color of the area of the slider that overlaps the region will also change, which makes the slider almost invisible. How can I avoid this?

My code is as follows:

    RECT channelRect, rectToFill;
    SendDlgItemMessage(hDlg, IDC_SLIDER1, TBM_GETCHANNELRECT, 0, (LPARAM)&channelRect);
    rectToFill = channelRect;
    rectToFill.right = (channelRect.left+channelRect.right)/2; // Fill the left half of the channel

    HDC hdc = GetDC(GetDlgItem(hDlg, IDC_SLIDER1)); 
    FillRect(hdc, &rectToFill, (HBRUSH) (COLOR_WINDOW+1));