[davon806]

AgF solid is brown.
How about its aqueous solution?Is it brown or colourless?
How to determine the colour of substance when it is dissolved in water without doing an experiment?
For example,anhydrous copper(II) sulphate dissolves in water and give a pale blue solution.
Any method to predict the colour of the resultant solution?

[Arkcon]

Only in rare situations is the aqueous solution very different in color from the solid.  Hard to even remember one case.  As for your specific case, AgF isn't soluble in water.

*EDIT*

No wait, it is somewhat soluble.  That was a surprise.

[davon806]

That means the solution of AgF is also brown?

[Sircodekill]

I'm not sure, but i believe that you simply cant predict the color of the solution. If your solid is dark red your solution probably will be dark red (depending on concentration of course) but if not... well, you can start the regular chinese-chemist talk which consist on basically: "I get explanations from my back" and start talking about d-d state transitions, H₂O ion solvation or what the hell you want.

The fact is you should never put your hand in the fire for these things.

[AWK]

Silver fluoride is well soluble in water - over 1 kg per liter of water.  Anhydrous AgF is colorless. The same concerns its solution in water. AgF dihydrate and tetrahydrate are yellow - see Wikipedia.