Solutions
We know the colour of 3 of the tiles:

An orange tile has 5 diamonds in nearby tiles. Since only 5 tiles are touching (1,3), all 5 of them must contain diamonds.
A green tile has 4 diamonds in adjacent tiles. Since only 5 tiles are touching (4,1), only one of them does not have a diamond. But (4,3) has the colour blue. Hence, only 2 of its adjacent tiles can have the diamonds. Since 3 of the tiles touching (4,1) are also touching (4,3), only two of them must have diamonds, and all the other tiles touching (4,3) must be empty. Thus, we can make the following table:

Under rule number 4, it has been given that any of the tiles will necessarily change its colour to one of the 5 given colours without violating any condition. What this implies is that all the tiles have only 1-5 diamonds in their vicinity, as changing to one of the colours without having 1-5 diamonds in the vicinity would violate the condition for that colour.
(5,5) has 3 adjacent tiles and 2 of them do not have any diamonds underneath. Hence, (4,5) necessarily contains a diamond, and he can deduce that (5,5) must necessarily be Red since only 1 tile in the vicinity has a diamond underneath. Hence, only on the basis of the 3 potions that the mysterious voice poured, Geralt has the following knowledge about the tiles:

If Geralt walks away with 8 diamonds, he did not find any more diamonds under the other tiles.
Hence, in the minimum condition, he uses 0 potions and only deduces the position of the diamonds using the potions the voice poured.
We can pour a potion on only 1 of the two tiles in the first column, as pouring on one gives us the information about the other. Similarly, we can pour only 1 potion in the two tiles in the second column.
Also, we can pour only 1 potion in the three tiles in column 5:
1. If we pour on (2,5): It tells us whether there is a diamond in the other two tiles or not. Hence, we cannot pour a potion on them.
2. If we pour on (1,5): It tells us whether there is a diamond in (2,5) or not. Hence, we can infer the colour of (3,5) too, and we cannot pour a potion on it.
3. If we pour on (3,5): It tells us whether there is a diamond in (2,5) or not. Hence, we can infer the colour of (1,5) too, and we cannot pour a potion on it.
Thus, Geralt can pour at most 3 more potions.
Thus, the minimum number of potions (a): 0.
The maximum number of potions (b): 3.
a+b = 0+3= 3.