Why are rain clouds dark?

Clouds are always light when viewed from above. The more they reflect light, the darker is the space below them. Larger droplets both reflect more light and absorb more, but from above it still looks totally white. When in a plane flying above the clouds we see a storm centre as a boil up of clouds, still white. The time that those boil-up clouds look dark is when we happen to see them with the sun shining through them, as when the sun is close to horizon.

when the particles formed it will be thickened and heavy,,, and the rain gonna start... when the clouds getting thick it overlapped the light from the sun,,, so we gonna see the difference from the other clouds it is more darker,,, thats all...

There was a cloud prog being shown a while ago and it was said the reason clouds are dark in colour is becaiuse they are heavy with water that carries dirt, sand, etc.

Sounds reasonable. I would have said that rain clouds are thicker and that each little particle of water absorbs some tiny amount of light. Thicker clouds have more water particles, and so absorb more light, and hence appear darker. But I suspect my theory only works if the sun is directly above the cloud. I had never heard that the water particles in rain clouds are bigger than in normal clouds, but that certainly makes sense -- they have to be in the process of getting bigger to form rain drops. And if that's the case, your theory makes more sense. It could also be that larger water particles have a different index of refraction. Maybe instead of absorbing the light, it is simply refracting it in a different direction.

Because they're loaded with humidity.

because they're full of rain water

Probably the latter. Rain clouds are unbelievably high, and appear like normal from above. But from below, there's often so much water with so much dust, carbon, etc. inside of the cloud, it blocks out the suns light.

the dark is the dirty rain waiting to fall.