### ContourPlot

Hope one of the three methods below work in Version 7.0.

**1.** You can use the options `MeshFunctions` and `Mesh` to add additional contours:

    ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
     PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
     ColorFunction -> "Rainbow", ImageSize -> 500, Axes -> True, 
     MeshFunctions -> {Sin[# - #2] + Cos[# + #2] &}, 
     Mesh -> {Thread[{{0., -1., 1.}, 
         Thread[Directive[{Green, Red, Blue}, Thick, Opacity[1]]]}]}]

[![enter image description here][1]][1]

**2.** Alternatively, post-process `ContourPlot` output to restyle selected contours:

    cp = ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
      PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
      ColorFunction -> "Rainbow", ImageSize -> 500, 
      Axes -> True]; 

    Replace[cp, {Tooltip[{d___, l__Line}, t : (0. | 1. | -1.)] :> {Thick, Opacity[1], 
        t /. {0. -> Green, -1. -> Red, 1. -> Blue, _ -> {d}},  Tooltip[{l}, t]}}, All]

[![enter image description here][2]][2]

**3.** Yet another method is to extract the contours from `cp` and redo `ContourPlot` using styled contours:

    contours = Cases[cp, Tooltip[_, t_] :> t, All]
>     {1.5, 1., 0.5, 0., -0.5, -1., -1.5}

    styledcontours = Join[ Thread[{{0., -1., 1.}, 
         Thread[Directive[{Green, Red, Blue}, Thick, Opacity[1]]]}], 
       Complement[contours, {0., -1., 1.}]];
    
    ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
     PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
     ColorFunction -> "Rainbow", ImageSize -> 500, Axes -> True, 
     Contours -> styledcontours]

[![enter image description here][3]][3]

### Plot3D

**1.** Add constant functions in the first argument of `Plot3D` corresponding to the desired levels, set their `PlotStyle` to `Opacity[0]` and use the option [`BoundaryStyle`](https://reference.wolfram.com/language/ref/BoundaryStyle.html) to set the directives for the intersection of the main surface with the added planes:

    Plot3D[{ 0., -1., 1., Sin[x - y] + Cos[x + y]}, {x, -10, 10}, {y, -10,
       10}, PlotPoints -> {30, 30}, 
     PlotRange -> {{-10, 10}, {-10, 10}, {-3, 3}}, 
     MeshFunctions -> (#3 &), 
     PlotStyle -> {Opacity[0], Opacity[0], Opacity[0], Automatic}, 
     ColorFunction -> "Rainbow", ImageSize -> 500, 
     BoundaryStyle -> {1 -> None, 2 -> None, 3 -> None, 
       {4, 1} -> Directive[Green, AbsoluteThickness[4], Opacity[1]], 
       {4, 2} -> Directive[Red, AbsoluteThickness[4], Opacity[1]],
       {4, 3} -> Directive[Blue, AbsoluteThickness[4], Opacity[1]]}, 
     Method -> {"RotationControl" -> "Globe"}, SphericalRegion -> True]

[![enter image description here][4]][4]


**2.** Post-process to restyle selected mesh lines:

    p3d = Plot3D[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
      PlotPoints -> {30, 30}, 
      PlotRange -> {{-10, 10}, {-10, 10}, {-3, 3}}, 
      MeshFunctions -> (#3 &), ColorFunction -> "Rainbow", 
      ImageSize -> 500, Method -> {"RotationControl" -> "Globe"}, 
      SphericalRegion -> True];
    
    Normal[p3d] /. Line[x_, ___] :> 
      {Round[x[[1, -1]], 0.1] /. Append[Thread[{0., -1., 1.} -> 
        Thread[Directive[{Green, Red, Blue}, Thick, Opacity[1]]]], _ -> {}],  Line[x]}

[![enter image description here][5]][5]


  [1]: https://i.sstatic.net/GZiAu.png
  [2]: https://i.sstatic.net/ymVMW.png
  [3]: https://i.sstatic.net/zrUtK.png
  [4]: https://i.sstatic.net/amvyb.png
  [5]: https://i.sstatic.net/81JbZ.png