Grevala is the largest dragon to have ever existed, and that includes her husband, the Brightness, the largest metallic dragon to ever exist. All the malignant dragons in the world see the Chromatic's evil abilities and hope to reach that power level. Many Chromatic Dragons don't really pray to the Chromatic but rather look up to her abilities and crave such power. If you're a Chromatic Dragon or Chromatic Dragonborn, you need not require a holy symbol, otherwise, you require a holy symbol made of a scale from every colour of chromatic dragon.
Grevala is less of a true dragon and more of a monstrosity, she keeps her old appearance of a massive red dragon, but she can change her form and add different parts to empower herself with other chromatic abilities. She can increase her size to make her breath weapons more powerful, increase her wingspan to make herself faster or even add different coloured dragon heads for different breath weapons. The Chromatic was sometimes depicted in her humanoid form pre-Era Martial, but ever since she is only ever depicted in her monstrosity form where she has every single possible enhancement she can have.