custom this color in your src/theme/variables.scss

available variables: https://ionicframework.com/docs/api/components/loading/LoadingController/#sass-variables

$loading-ios-background: transparent;
$loading-md-background: $loading-ios-background;
$loading-wp-background: $loading-ios-background;

to remove box-shadow on android, add one more variable:

$loading-md-box-shadow: none;

or add your class to cssClass:

this.loading = this.loadingCtrl.create({
  content: '',
  spinner: 'dots',
  cssClass: 'my-loading-class'
});

and style:

============================

UPDATE: IONIC 3

ion-loading.my-loading-class {
  .loading-wrapper {
    background: transparent;
    box-shadow: none;
  }
}

============================

IONIC 2

.loading-ios,
.loading-md,
.loading-wp {
  .my-loading-class {
    background-color: transparent;
    box-shadow: none;
  }
}

Tiep Phan's solution didn't work for me, so please see the example below for a working solution as of Ionic version 3.19.0:

app.component.ts

const LOADING = this.loadingCtrl.create({
  cssClass: 'transparent',
});

app.scss

ion-loading.transparent {
  .loading-wrapper {
    background: transparent;
    box-shadow: none;

    .spinner-crescent circle {
      stroke-width: 8px;
    }
  }
}

Please note that I have optionally increased the stroke width of the Android loading spinner to make it slightly more visible.

This works for me with Ionic 4 and 5 for Android and iOS (using mode:'md'):

import { Injectable } from '@angular/core';
import { LoadingController } from '@ionic/angular';
import { LoadingOptions } from '@ionic/core';

@Injectable({
  providedIn: 'root'
})
export class LoadingService {

  constructor(public loadingController: LoadingController) { }

  async present() {
// Dismiss all pending loaders before creating the new one
await this.dismiss();

let options: LoadingOptions = {
  message: '<ion-img src="/assets/imgs/spinner.svg" alt="loading..."></ion-img>',
  cssClass: 'custom-loading',
  translucent: true,
  showBackdrop: true,
  spinner: null,
  mode: 'md',
  keyboardClose: true
};

await this.loadingController  
  .create(options)
  .then(res => {
    res.present();
  });
}

  /**
   * Dismiss all the pending loaders, if any
   */
  async dismiss() {
    while (await this.loadingController.getTop() !== undefined) {
      await this.loadingController.dismiss();
    }
  }
}

and then in global.scss:

.custom-loading {
    --background: none;
   
    .loading-wrapper{
        box-shadow: none !important;
        -webkit-box-shadow: !important;
    }
 }