Hi Sudiptaman,
Under the Sprite Renderer stack of your emitter, make sure you have alignment set to Velocity Aligned.
In regards to changing the color and making the effect more subtle, try this:
Under Initialize Particle, you can set the Color and Alpha (A).
Color is self explanatory, Alpha will control the opacity of the rain drop. A value of 1.0 is completely visible, a value of 0.0 is completely invisible. Try a value of .5 or .75 to see if it helps visually make the effect more subtle.
Is this what you were looking for? If not, can you rephrase your question with more detail?
Thank you!