A standard dal tends to be yellow or orange, using red or yellow lentils. I had a packet of urid dal lurking about, to be used up. I’ve never made Dal Makhani, but looking it up, it turns out the correct lentil for the recipe is urid (or urad). Urid dal is sometimes described as creamy or even slimy. (It’s amazing how many cultures, particularly Africa, love slimy food).
A scoop of urid/urad dal is often added ‘tempered’ to Indian dishes to add texture and nutrition. I love it in lemon rice for instance.