In D. H. Lawrence's poem "Snake," repetition is used throughout for emphasis. In the line your question refers to, the word "earth" is appended twice to another descriptor—a color descriptor. This results in two compound adjectives, "earth-brown" and "earth golden". Lawrence then in fact repeats the word "earth" a third time, in the following line. There clearly, then, must be a reason for such concentration on the earth-like qualities of the snake.
Lawrence describes the snake as being both brown and golden, in the same way in which the earth is brown and golden—particularly in a place which is so proximate to a volcano. Thinking of the snake as being like the earth makes us think instantly of the snake as having come from the earth—from, indeed, the center of the earth—which is, of course, impossible but still creates a vivid image. The snake being earth-colored also emphasizes the fact that it has been able to appear in the garden unexpectedly; it is well-camouflaged. It is also, being a creature of the earth, something which belongs here perhaps more than the speaker does, something that is suggested elsewhere in the poem (as when the speaker describes himself as a "second comer") when he approaches the trough at which the snake is drinking.
Lawrence's poem is a very vivid one in terms of its descriptors. The snake is "golden" for a reason: Lawrence seems to set it on a pedestal above the mere humans who enter the outside space.