The color on the shelf is a clue to the texture in the pan. Split red lentils usually soften and break down, which is welcome in soup, a thick sauce, or a spoonable filling. Whole brown or green lentils are more useful when you want the pieces to remain visible.
That difference matters when substituting. A salad designed around firm cooked lentils will not behave the same way with red lentils, even if the weight is identical. For a blended soup, the swap may be much less important.
Check the packet for rinsing and cooking instructions, and begin tasting before the longest suggested time. Age and variety affect how quickly lentils soften. Drain when they are right for the dish, rather than keeping them on the heat because another ingredient is not ready yet.