The setBlendConstant() method of the GPURenderPassEncoder interface sets the constant blend color and alpha values used with "constant" and "one-minus-constant" blend factors (as set in the descriptor of the GPUDevice.createRenderPipeline() method, in the blend property). 
 
 color  -  
An object or array representing the color to use when blending — the r, g, b, and a components are represented as floating point numbers between 0.0 and 1.0.
 What follows is an object example:
 
const color = { r: 0.0, g: 0.5, b: 1.0, a: 1.0 };
  The array equivalent would look like this:
 
const color = [0.0, 0.5, 1.0, 1.0];
    
  Note: If a setBlendConstant() call is not made, the blend constant color value defaults to (0, 0, 0, 0) for each render pass.
  
 
const passEncoder = commandEncoder.beginRenderPass(renderPassDescriptor);
passEncoder.setPipeline(renderPipeline);
passEncoder.setVertexBuffer(0, vertexBuffer);
passEncoder.setBlendConstant([1.0, 0.0, 0.0, 1.0]);
passEncoder.draw(3);
passEncoder.end();