in LW the color target should be created in SRGB

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipeline.cs

             }
 
             RenderTextureDescriptor colorRTDesc = new RenderTextureDescriptor(rtWidth, rtHeight, m_ColorFormat, kDepthStencilBufferBits);
+            colorRTDesc.sRGB = true;
             colorRTDesc.msaaSamples = msaaSamples;
             colorRTDesc.enableRandomWrite = false;