**3D**

Set the `Mesh` to 
```
 Mesh -> {{{-1, {Thick, Red}}, {0, {Thick, Green}}, {1, {Thick, 
        Blue}}}}
```

```
a = Plot3D[
   Sin[x - y] + 
    Cos[x + y],(*this function is just for the MWE*){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];
b = 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 &), 
   Mesh -> {{{-1, {Thick, Red}}, {0, {Thick, Green}}, {1, {Thick, 
        Blue}}}}, PlotStyle -> None];
Show[a, b]
```

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


**2D**

Set `Contours` 

```
Contours -> {{-1, {Thick, Red}}, {0, {Thick, Green}}, {1, {Thick, 
       Blue}}}
```

```
aa = ContourPlot[
   Sin[x - y] + 
    Cos[x + y],(*this function is just for the MWE*){x, -10, 
    10}, {y, -10, 10}, PlotPoints -> {30, 30}, 
   PlotRange -> {{-10, 10}, {-10, 10}}, ColorFunction -> "Rainbow", 
   ImageSize -> 500, MeshFunctions -> (#3 &), Mesh -> None, 
   ContourStyle -> None, ContourShading -> Automatic];
bb = ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
   PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
   ImageSize -> 500, ContourShading -> None, 
   Contours -> {{-1, {Thick, Red}}, {0, {Thick, Green}}, {1, {Thick, 
       Blue}}}];
Show[aa, bb]
```

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


  [1]: https://i.sstatic.net/vTF1I.png
  [2]: https://i.sstatic.net/vbxPH.png