Compute shader: Why colors inverse due to proximity to the object?

Hello everybody,

I'm working with an existing shader, which simulates fluids. I need the fluids to look as if they are "sandy", so I've added a normal map to the shader.

             float4 frag(v2f IN) : COLOR
             {
                 half4 bump = tex2D(_BumpMap, IN.uv);
                 half2 distorion = UnpackNormal(bump).rg;
 
                 IN.uvgrab.xy += distorion * _Magnitude;
 
                 fixed4 col = tex2Dproj(_GrabTexture, UNITY_PROJ_COORD(IN.uvgrab));
 
                 int x = (int)(IN.uv.x*_Size.x);
                 int y = (int)(IN.uv.y*_Size.y);
 
                 return _ColourRamp[clamp(_Particles[y * _Size.x + x], 0.0, 255.0)] * col;
             }    

In this frag function I apply the normal map together with the colors, by multiplying the returned color by the normal col variable.

This is the outcome:

You can see two images of the same object but with different colors. The first (red tinted) image is taken from a very close angle - and it actually looking as expected.

The second image is taken from a far angle, and is looking like the colors are inverse.

Why do I get this color inversion behavior?

Please help!