Well... You can't have different sparks regarding to replays and playing lol.
To change them, open the "particle.cfg" file located at the data folder of your VC game root. Search for the "SPARK_SMALL" line. Now, follow this instructions:
1.- Open Paint
2.- Choose the colour you want
3.- Copy those RGB values.
4.- Replace the SPARK_SMALL first 3 values for the values that are in Paint.
You now have changed the first spark color (Fade In).
Do the same steps for the other three values to change the second spark color (Fade Out).