Congratulations on identifying one of the central contrasts in this incredible poem. Of course, Tennyson seems to hae deliberately created a whole set of contrasting images that deliberately present the Lady of Shallot and Sir Lancelot in contrast with each other. Notice how, up until the entrance of Sir Lancelot, we associate the Lady of Shallot with a world of shadows and greyness. The second stanza, for example, presents us with a rather bleak, austere and colourless setting:
Four grey walls, and four grey towers,
Overlook a space of flowers,
And the silent isle imbowers
The Lady of Shallot.
The Lady of Shallot herself characterises her life as consisting of "shadows" that she is tired of.
By contrast, note how Sir Lancelot is introduced:
A bowshot from her bower eaves,
He rode between the barley sheaves,
The sun came dazzling through the leaes,
And flamed upon the brazen greaves
Of bold Sir Lancelot.
We associate his character with action, movement and colour, compared to the passive nature of the Lady of Shallot. Note how the sun "dazzles" as it reflects of the "brazen greaves" of Sir Lancelot, and how he is presented immediately as being associated with speed. Tennyson thus creates a contrast between life and a pale imitation of life, or perhaps between life and death.