Sieve 80 g ground almonds into a bowl, and discard any bits of almonds or skin which remain in the sieve. You can use "white" ground almonds (without skin) or "grey" (with skin), it's a question of taste and appearence of the finished macarons. In my opinion, "grey" almond macarons have more flavour, but less clear colour (natural, green, yellow, pink, etc...).
Sieve 150 g icing sugar into the same bowl. These siftings (and that which follows) are to obtain a fine mixture of ground almonds and icing sugar, without any small lumps.
Then mix almonds and icing sugar with a whisk, to avoid lumps reforming.
Put 2 egg whites, a few drops of lemon juice and a pinch of salt in a mixer bowl. Note: If you have several egg-whites in a bowl, and don't know how many, remember that one white egg is about 35 g. Beat on high speed until stiff adding the tablespoon of sugar halfway through.
Add the 6 drops of colouring, and beat a few seconds more to mix thoroughly.
We do this, instead of adding colouring before beating, to keep the colour stronger.
Sieve the icing sugar and almonds mixture onto the egg-whites.
Tip the bowl slightly and, with a maryse or a spatula (a maryse is much better), mix gently, turning over from top to bottom rather than round.
This is the first tricky stage: you should work the mixture, the aim is not to obtain a light mixture (as for a mousse for example)...
...but something slighly runny and shiny. This process is called "macaroner" in French(to macaron, perhaps?).
With a forcing bag (or a teaspoon, but it's much more difficult), form small heaps of mixture on cooking parchment or silicon paper on a baking sheet about the size of a 2 euro coin (1"). Leave a reasonable space between heaps, because they will spread out a little.
Tap the baking sheet gently on the work surface to spread out the heaps, and to round them (to remove the small point left by raising the forcing bag).
See the photos for what happens after tapping the baking sheet.
Leave at room temperature for 30 minutes to one hour, for the heaps to "form a crust". This is the 2nd tricky stage: this time of "forming crust" is important for macarons which are well rounded, and do not split during cooking.
To test if your macarons are sufficiently "crusted", touch a top lightly with your finger: if the dough does not stick to your finger, the macarons are ready.
Then preheat the oven to 160°C or 320°F, and cook for 12 to 15 minutes. For some mysterious reason I have found that macarons rise best if they are on two baking sheets (one on top of the other) instead of a single one. Turn the baking sheets (front to back) after 6 minutes for even cooking.
Leave to cool completely before removing the macarons, and place on a wire rack to await filling. This is the 3rd tricky stage: You should not overcook the macarons, which should stay moist in the middle. Watch the coloration: if you have used food colouring, they should stay that colour and not become brown.
Pair-up the macarons, i.e. by putting together a "top" and a "bottom" of about the same diameter. Then place "top" and "bottom" beside each other.