EDIT: Its supposed to look like the first image
Sorry if this is answered already i cant find the answer on the internet but im having a problem rendering this scene
This was the work around i found that works
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Uncheck "Object Data properties > Layers > Use Lights" for each layer.
This option was added in Blender 2.83. When on, GP object colors are affected by all lights. This means that if there are not enough light sources, the GP object will be dark. When off, it will render the same color as selected in the Color Picker.
See Official manual: Object — Blender Manual
Your problem is probably due to default settings of default brush. Its strength is by default 0.6. It means that strokes drawn with that brush will have an alpha multiplied by 0.6.
To solve the problem, go to sculpt mode and use the strength brush to give a value of 1 to strength of drawn strokes. Or you can add an opacity modifier to BP object and set its factor above 1. It is a pity that there is no global adjustment setting or operator to solve that kind of problem.
These defaults are very misleading.