In Lama Zopa's Advice Book (
http://dorjeshugden.com/wp/?p=1166), he gives advice on how dangerous it is if we were to have negative thoughts about our teachers. This advice is the same across all the High lamas and is in the Lamrim and scriptures such as the 50 verses of Guru Devotion. Whether yellow or beige will have its own merits but once we choose yellow OR beige, let's just stick to it!!
This below is an extract from Lama Zopa's advice book:
In Lama Tsongkhapa’s Lam-rim Chen-mo (The Great Treatise on the Stages of the Path to Enlightenment), it says, “Even simply generating non-devotional thought, to think of the guru as an ordinary person, is an obstacle.” So, you can imagine what it is like if you generate heresy and anger toward your gurus and criticize them with a negative mind. This is creating the heaviest obstacle to realizations on the path to enlightenment, and the heaviest obstacle even to this life’s happiness, as well as to future lives’ happiness. There is no need to mention liberation or enlightenment.
The other thing is that in this life, by generating heretical thoughts or anger toward the guru, one experiences heavy sufferings, especially sickness, and many catastrophes, one after the other. Also, when you die, it is in such a terrible way. It is very terrifying, not peaceful, and there is the most unbearable suffering in the hell realms. Not only that, but it lasts for such a length of time, for eons.
Please read the section on guru devotion in Pabongka Dechen Nyingpo’s Liberation in the Palm of Your Hand. Also, read the section on guru devotion in Lama Tsongkhapa’s Lam-rim Chen-mo. Please study other Dharma books, at least these two texts and the sections that I have mentioned.
In the teaching Commentary on the Difficult Points of the “Black Enemy of Yama,” (Tib: Dra-nag-ka-drel) (see Steps on the Path to Enlightenment, Volume 1, page 157) it is said that after having made a Dharma connection, receiving teachings (even four words), with a recognition of a guru-disciple relationship (the other person is your guru and you are his or her disciple), if you forget to devote yourself to that person as a guru, or if you don’t regard them as your guru, you will be born as a dog for one hundred lifetimes, and then you will be born into a lower rank. His Holiness Zong Rinpoche says one is born as a scorpion instead of in a lower rank.
If you did create negative karma with your teachers, please confess whatever heresy, anger, or criticism you have committed against them. Confess as quickly as possible.
If death comes before your confession, your future lives will be so difficult. Not only will you be unable to meet the guru, but you will also be unable to meet the Dharma. Not only that, but you will experience unimaginable suffering for an incredible amount of time.
The Fifth Dalai Lama advised how to practice guru devotion in this kind of situation. When you have difficulty with gurus, seeing mistakes in their actions, then think that this is the view of your own hallucinated mind. All these appearances are your own mistakes manifested as the guru’s holy actions. Recognize it as your own mistake and abandon it, like a poison.