You can induce weak/artificial synesthesia on yourself, you cannot induce strong synesthesia on yourself.
The type of synesthesia you describe is the same type that Ramachandran mentions when hypothesizing that synesthesia is not a legitimate sensory experience:
"Could we be absolutely sure that this wasn't happening because early in kindergarten she had repeatedly seen a red seven on her refrigerator door?"
This is simply an association, like remembering the qualities of bacon when it is smelled. True, strong synesthesia is not something that can be induced by association. It is not an association within how they think of something, but a secondary modal experience that is triggered automatically when they experience the first modal experience.
The modern test for synesthesia, the stroop test, perhaps demonstrates this best. In a normal stroop test, participants are asked to recognize a color word that is in a different color (the word "blue" on a red font). The meaning of the word and the color of the word cause a temporary automatic conflict in your brain, and your results are slower than if they matched or were all black. If you ask a grapheme-visual synesthese to recognize numbers that disagree with the colors they see it as, their responses will slow. If you were to undergo this memorization training, your responses would not slow, the association you have is memory based and would occur after recognition processing.
To expand on the differences between weak synesthesia and strong synesthesia
Weak synesthesia is association or metaphor. You do not experience the secondary meaning, you simply think of it. It can sometimes seem very close to experiencing it, but it is still separate. If you are thinking of red after seeing a 7, someone could distract you or you could willingly think of another color equally.
Strong synesthesia is an automatic inappropriate modal response. A modal response is the response of one distinct sense to an input. For most people, this is the normal things, sound and hearing, scent and smell... For a synesthese the color they see on a number or while listening to music is exactly as real as the number or music itself. Someone attempting to emulate this may imagine the number they see in their mind and then think of a color. To the synesthese, not only is the synesthetic color on the number, but so is the actual color of the number. It does not overwrite the actual input, it is simply additional input. It is 'inappropriate' because it is a response that is not supposed to happen for that input. It is automatic in the sense that it cannot be ignored, blocked out, distracted from, changed in any way, or otherwise consciously altered. Trying to do so would be equivalent to trying to convince yourself you do not see a cup. It won't work, your automatic senses will still tell you there is a cup.
For more information, I would recommend any of the following:
The Man Who Tasted Shapes by Dr. Cytowic is the original modern research on synesthesia, though the hypothesis Cytowic puts forth is not held very highly anymore.
The Tell-Tale Brain by V.S. Ramachandran has a section on Synesthesia incorporating both real-world examples and the most up-to-date research on the matter in a very readable way.
Varieties of Anomalous Experience is not as up-to-date as Ramachandran but cites specific studies and research in more detail.
