Rosé is regarded as a simple fun wine, but actually, it is a complex wine to make and it requires high attention to details for winemakers.
Most of rosés are made from black grapes, therefore there is no white wine added to make it pink (Obviously there is always exceptions as La Jara rosé)
If you harvest black grapes and decide to remove the skin, as a result you would have a wine white. For instance, most champagnes are made this way.
For Rosé, winemakers keep the skin with the juice but for a very short period of time, then they will discard the skin.
If by mistake, they forget the skin, we will have as a result, a red wine.
In consequence, the myth that a darker Rosé is sweeter is wrong. The sweetness is not connected with the colour.
To be a bit more technical there is three main ways to make a rose :
The rosé from direct press : Black grapes are gently directly pressed to collect a very light pale colour juice. This is the way to make a cote de provence rosé. (Domaine Terre de Mistral)
Rose of maceration : After pressing the grapes, the skin will stay in contact with the juice, as a result, the wine is darker. This is the way to make a Tavel or cerasuolo d’abruzzo.
Skin from grapes bring colour, but also provide tannins and flavours. Therefore a rose of maceration will usually be fuller and richer in flavours.
Rosé de Saigné : This technique is a bit less common. Winemaker will extract some juice from the must. The removed juice will often look like a dark rosé. This technique will also allow the rest of the must to be more concentrated and therefore to produce more powerful and tannic red wines.